smtp: ignore HELO error in QUIT

Merge from net/smtp/smtp.go

Obtained from:	2ef8e41f9543478a51a0147a735e4415737de09f

.github/workflows/codeql-analysis.yml

@@ -1,55 +1,64 @@
+# For most projects, this workflow file will not need changing; you simply need
+# to commit it to your repository.
+#
+# You may wish to alter this file to override the set of languages analyzed,
+# or to provide custom queries or build logic.
+#
+# ******** NOTE ********
+# We have attempted to detect the languages in your repository. Please check
+# the `language` matrix defined below to confirm you have the correct set of
+# supported CodeQL languages.
+#
 name: "CodeQL"
 
 on:
   push:
-    branches: [master]
   pull_request:
     # The branches below must be a subset of the branches above
     branches: [master]
   schedule:
     - cron: '0 15 * * 5'
 
+permissions:
+  contents: read
+
 jobs:
   analyze:
     name: Analyze
     runs-on: ubuntu-latest
+    permissions:
+      actions: read
+      contents: read
+      security-events: write
 
     strategy:
       fail-fast: false
       matrix:
-        # Override automatic language detection by changing the below list
+        language: [ 'go' ]
-        # Supported options are ['csharp', 'cpp', 'go', 'java', 'javascript', 'python']
-        language: ['go']
-        # Learn more...
-        # https://docs.github.com/en/github/finding-security-vulnerabilities-and-errors-in-your-code/configuring-code-scanning#overriding-automatic-language-detection
 
     steps:
-    - name: Checkout repository
+    - name: Harden Runner
-      uses: actions/checkout@v3
+      uses: step-security/harden-runner@0634a2670c59f64b4a01f0f96f84700a4088b9f0 # v2.12.0
       with:
-        # We must fetch at least the immediate parents so that if this is
+        egress-policy: audit
-        # a pull request then we can checkout the head.
-        fetch-depth: 2
 
-    # If this run was triggered by a pull request event, then checkout
+    - name: Checkout repository
-    # the head of the pull request instead of the merge commit.
+      uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
-    - run: git checkout HEAD^2
-      if: ${{ github.event_name == 'pull_request' }}
 
     # Initializes the CodeQL tools for scanning.
     - name: Initialize CodeQL
-      uses: github/codeql-action/init@v2
+      uses: github/codeql-action/init@fca7ace96b7d713c7035871441bd52efbe39e27e # v3.28.19
       with:
         languages: ${{ matrix.language }}
         # If you wish to specify custom queries, you can do so here or in a config file.
-        # By default, queries listed here will override any specified in a config file. 
+        # By default, queries listed here will override any specified in a config file.
         # Prefix the list here with "+" to use these queries and those in the config file.
         # queries: ./path/to/local/query, your-org/your-repo/queries@main
 
     # Autobuild attempts to build any compiled languages  (C/C++, C#, or Java).
     # If this step fails, then you should remove it and run the build manually (see below)
     - name: Autobuild
-      uses: github/codeql-action/autobuild@v2
+      uses: github/codeql-action/autobuild@fca7ace96b7d713c7035871441bd52efbe39e27e # v3.28.19
 
     # ℹ️ Command-line programs to run using the OS shell.
     # 📚 https://git.io/JvXDl
@@ -63,4 +72,4 @@ jobs:
     #   make release
 
     - name: Perform CodeQL Analysis
-      uses: github/codeql-action/analyze@v2
+      uses: github/codeql-action/analyze@fca7ace96b7d713c7035871441bd52efbe39e27e # v3.28.19

.github/workflows/dependency-review.yml

@@ -0,0 +1,27 @@
+# Dependency Review Action
+#
+# This Action will scan dependency manifest files that change as part of a Pull Request,
+# surfacing known-vulnerable versions of the packages declared or updated in the PR.
+# Once installed, if the workflow run is marked as required,
+# PRs introducing known-vulnerable packages will be blocked from merging.
+#
+# Source repository: https://github.com/actions/dependency-review-action
+name: 'Dependency Review'
+on: [pull_request]
+
+permissions:
+  contents: read
+
+jobs:
+  dependency-review:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Harden Runner
+        uses: step-security/harden-runner@0634a2670c59f64b4a01f0f96f84700a4088b9f0 # v2.12.0
+        with:
+          egress-policy: audit
+
+      - name: 'Checkout Repository'
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+      - name: 'Dependency Review'
+        uses: actions/dependency-review-action@da24556b548a50705dd671f47852072ea4c105d9 # v4.7.1

.github/workflows/go.yml

@@ -1,13 +1,21 @@
 name: Go
 on: [push, pull_request]
+permissions:
+  contents: read
+
 jobs:
 
   build:
     name: Build
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - name: Harden Runner
-    - uses: actions/setup-go@v4
+      uses: step-security/harden-runner@0634a2670c59f64b4a01f0f96f84700a4088b9f0 # v2.12.0
+      with:
+        egress-policy: audit
+
+    - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+    - uses: actions/setup-go@d35c59abb061a4a6fb18e82ac0862c26744d6ab5 # v5.5.0
       with:
         go-version: 'stable'
 

.github/workflows/release.yaml

@@ -1,9 +1,12 @@
 name: Release Go Binaries
 
-on: 
+on:
   release:
     types: [created]
 
+# Declare default permissions as read only.
+permissions: read-all
+
 jobs:
   releases-matrix:
     name: Release Go Binary
@@ -12,19 +15,28 @@ jobs:
       matrix:
         goos: [freebsd, linux, windows]
         goarch: [amd64, arm64]
+    permissions:
+        contents: write
+        packages: write
+
     steps:
-    - uses: actions/checkout@v3
+    - name: Harden Runner
+      uses: step-security/harden-runner@0634a2670c59f64b4a01f0f96f84700a4088b9f0 # v2.12.0
+      with:
+        egress-policy: audit
+
+    - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
     
     - name: Set APP_VERSION env
       run: echo APP_VERSION=$(echo ${GITHUB_REF} | rev | cut -d'/' -f 1 | rev ) >> ${GITHUB_ENV}
     - name: Set BUILD_TIME env
       run: echo BUILD_TIME=$(date) >> ${GITHUB_ENV}
       
-    - uses: wangyoucao577/go-release-action@v1
+    - uses: wangyoucao577/go-release-action@481a2c1a0f1be199722e3e9b74d7199acafc30a8 # v1.53
       with:
         github_token: ${{ secrets.GITHUB_TOKEN }}
         goos: ${{ matrix.goos }}
         goarch: ${{ matrix.goarch }}
-        goversion: "1.20"
+        goversion: "1.24"
         extra_files: LICENSE README.md smtprelay.ini
         ldflags: -s -w -X "main.appVersion=${{ env.APP_VERSION }}" -X "main.buildTime=${{ env.BUILD_TIME }}"

.github/workflows/scorecards.yml

@@ -0,0 +1,81 @@
+# This workflow uses actions that are not certified by GitHub. They are provided
+# by a third-party and are governed by separate terms of service, privacy
+# policy, and support documentation.
+
+name: Scorecard supply-chain security
+on:
+  # For Branch-Protection check. Only the default branch is supported. See
+  # https://github.com/ossf/scorecard/blob/main/docs/checks.md#branch-protection
+  branch_protection_rule:
+  # To guarantee Maintained check is occasionally updated. See
+  # https://github.com/ossf/scorecard/blob/main/docs/checks.md#maintained
+  schedule:
+    - cron: '20 7 * * 2'
+  push:
+    branches: ["master"]
+
+# Declare default permissions as read only.
+permissions: read-all
+
+jobs:
+  analysis:
+    name: Scorecard analysis
+    runs-on: ubuntu-latest
+    permissions:
+      # Needed to upload the results to code-scanning dashboard.
+      security-events: write
+      # Needed to publish results and get a badge (see publish_results below).
+      id-token: write
+      contents: read
+      actions: read
+      # To allow GraphQL ListCommits to work
+      issues: read
+      pull-requests: read
+      # To detect SAST tools
+      checks: read
+
+    steps:
+      - name: Harden Runner
+        uses: step-security/harden-runner@0634a2670c59f64b4a01f0f96f84700a4088b9f0 # v2.12.0
+        with:
+          egress-policy: audit
+
+      - name: "Checkout code"
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          persist-credentials: false
+
+      - name: "Run analysis"
+        uses: ossf/scorecard-action@05b42c624433fc40578a4040d5cf5e36ddca8cde # v2.4.2
+        with:
+          results_file: results.sarif
+          results_format: sarif
+          # (Optional) "write" PAT token. Uncomment the `repo_token` line below if:
+          # - you want to enable the Branch-Protection check on a *public* repository, or
+          # - you are installing Scorecards on a *private* repository
+          # To create the PAT, follow the steps in https://github.com/ossf/scorecard-action#authentication-with-pat.
+          # repo_token: ${{ secrets.SCORECARD_TOKEN }}
+
+          # Public repositories:
+          #   - Publish results to OpenSSF REST API for easy access by consumers
+          #   - Allows the repository to include the Scorecard badge.
+          #   - See https://github.com/ossf/scorecard-action#publishing-results.
+          # For private repositories:
+          #   - `publish_results` will always be set to `false`, regardless
+          #     of the value entered here.
+          publish_results: true
+
+      # Upload the results as artifacts (optional). Commenting out will disable uploads of run results in SARIF
+      # format to the repository Actions tab.
+      - name: "Upload artifact"
+        uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4.6.2
+        with:
+          name: SARIF file
+          path: results.sarif
+          retention-days: 5
+
+      # Upload the results to GitHub's code scanning dashboard.
+      - name: "Upload to code-scanning"
+        uses: github/codeql-action/upload-sarif@fca7ace96b7d713c7035871441bd52efbe39e27e # v3.28.19
+        with:
+          sarif_file: results.sarif

README.md

@@ -1,6 +1,7 @@
 # smtprelay
 
 [![Go Report Card](https://goreportcard.com/badge/github.com/decke/smtprelay)](https://goreportcard.com/report/github.com/decke/smtprelay)
+[![OpenSSF Scorecard](https://img.shields.io/ossf-scorecard/github.com/decke/smtprelay?label=openssf%20scorecard&style=flat)](https://scorecard.dev/viewer/?uri=github.com/decke/smtprelay)
 
 Simple Golang based SMTP relay/proxy server that accepts mail via SMTP
 and forwards it directly to another SMTP server.

SECURITY.md

@@ -0,0 +1,51 @@
+# smtprelay Security Policy
+
+This document outlines security procedures and general policies for the
+smtprelay project.
+
+## Supported Versions
+
+The latest release is the only supported release.
+
+
+## Disclosing a security issue
+
+The smtprelay maintainers take all security issues in the project seriously.
+Thank you for improving the security of the project! We appreciate your
+dedication to responsible disclosure and will make every effort to acknowledge
+your contributions.
+
+smtprelay leverages GitHub's private vulnerability reporting.
+
+To learn more about this feature and how to submit a vulnerability report,
+review [GitHub's documentation on private reporting](https://docs.github.com/code-security/security-advisories/guidance-on-reporting-and-writing-information-about-vulnerabilities/privately-reporting-a-security-vulnerability).
+
+Here are some helpful details to include in your report:
+
+- a detailed description of the issue
+- the steps required to reproduce the issue
+- versions of the project that may be affected by the issue
+- if known, any mitigations for the issue
+
+A maintainer will acknowledge the report within three (3) business days, and
+will send a more detailed response within an additional three (3) business days
+indicating the next steps in handling your report.
+
+After the initial reply to your report, the maintainers will endeavor to keep
+you informed of the progress towards a fix and full announcement, and may ask
+for additional information or guidance.
+
+## Vulnerability management
+
+When the maintainers receive a disclosure report, they will coordinate the
+fix and release process, which involves the following steps:
+
+- confirming the issue
+- determining affected versions of the project
+- auditing code to find any potential similar problems
+- preparing fixes for all releases under maintenance
+
+## Suggesting changes
+
+If you have suggestions on how this process could be improved please submit an
+issue or pull request.

cmd/README.md

@@ -1,6 +0,0 @@
-
-To run the hasher, do like this
-
-```bash
-$ go run hasher.go hunter2
-```

cmd/hasher.go

@@ -1,22 +0,0 @@
-package main
-
-import (
-	"fmt"
-	"os"
-
-	"golang.org/x/crypto/bcrypt"
-)
-
-func main() {
-	if len(os.Args) != 2 {
-		fmt.Fprintln(os.Stderr, "Usage: hasher PASSWORD")
-		os.Exit(1)
-	}
-	password := os.Args[1]
-
-	hash, err := bcrypt.GenerateFromPassword([]byte(password), bcrypt.DefaultCost)
-	if err != nil {
-		fmt.Fprintln(os.Stderr, "Error generating hash: %s", err)
-	}
-	fmt.Println(string(hash))
-}

config.go

@@ -45,6 +45,7 @@ var (
 	allowedUsers     = flagset.String("allowed_users", "", "Path to file with valid users/passwords")
 	command          = flagset.String("command", "", "Path to pipe command")
 	remotesStr       = flagset.String("remotes", "", "Outgoing SMTP servers")
+	strictSender     = flagset.Bool("strict_sender", false, "Use only SMTP servers with Sender matches to From")
 
 	// additional flags
 	_           = flagset.String("config", "", "Path to config file (ini format)")

go.mod

@@ -2,20 +2,20 @@ module github.com/decke/smtprelay
 
 require (
 	github.com/chrj/smtpd v0.3.1
-	github.com/google/uuid v1.3.0
+	github.com/google/uuid v1.6.0
-	github.com/peterbourgon/ff/v3 v3.3.2
+	github.com/peterbourgon/ff/v3 v3.4.0
 	github.com/sirupsen/logrus v1.9.3
-	github.com/stretchr/testify v1.8.4
+	github.com/stretchr/testify v1.10.0
-	golang.org/x/crypto v0.10.0
+	golang.org/x/crypto v0.38.0
 )
 
 require (
 	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/kr/pretty v0.3.1 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
-	golang.org/x/sys v0.9.0 // indirect
+	golang.org/x/sys v0.33.0 // indirect
 	gopkg.in/check.v1 v1.0.0-20201130134442-10cb98267c6c // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
 )
 
-go 1.20
+go 1.24.3

go.sum

@@ -4,8 +4,8 @@ github.com/creack/pty v1.1.9/go.mod h1:oKZEueFk5CKHvIhNR5MUki03XCEU+Q6VDXinZuGJ3
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
-github.com/google/uuid v1.3.0 h1:t6JiXgmwXMjEs8VusXIJk2BXHsn+wx8BZdTaoZ5fu7I=
+github.com/google/uuid v1.6.0 h1:NIvaJDMOsjHA8n1jAhLSgzrAzy1Hgr+hNrb57e+94F0=
-github.com/google/uuid v1.3.0/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
+github.com/google/uuid v1.6.0/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
 github.com/kr/pretty v0.2.1/go.mod h1:ipq/a2n7PKx3OHsz4KJII5eveXtPO4qwEXGdVfWzfnI=
 github.com/kr/pretty v0.3.1 h1:flRD4NNwYAUpkphVc1HcthR4KEIFJ65n8Mw5qdRn3LE=
 github.com/kr/pretty v0.3.1/go.mod h1:hoEshYVHaxMs3cyo3Yncou5ZscifuDolrwPKZanG3xk=
@@ -13,8 +13,8 @@ github.com/kr/pty v1.1.1/go.mod h1:pFQYn66WHrOpPYNljwOMqo10TkYh1fy3cYio2l3bCsQ=
 github.com/kr/text v0.1.0/go.mod h1:4Jbv+DJW3UT/LiOwJeYQe1efqtUx/iVham/4vfdArNI=
 github.com/kr/text v0.2.0 h1:5Nx0Ya0ZqY2ygV366QzturHI13Jq95ApcVaJBhpS+AY=
 github.com/kr/text v0.2.0/go.mod h1:eLer722TekiGuMkidMxC/pM04lWEeraHUUmBw8l2grE=
-github.com/peterbourgon/ff/v3 v3.3.2 h1:2J07/5/36kd9HYVt42Zve0xCeQ+LLRIvoKrt6sAZXJ4=
+github.com/peterbourgon/ff/v3 v3.4.0 h1:QBvM/rizZM1cB0p0lGMdmR7HxZeI/ZrBWB4DqLkMUBc=
-github.com/peterbourgon/ff/v3 v3.3.2/go.mod h1:zjJVUhx+twciwfDl0zBcFzl4dW8axCRyXE/eKY9RztQ=
+github.com/peterbourgon/ff/v3 v3.4.0/go.mod h1:zjJVUhx+twciwfDl0zBcFzl4dW8axCRyXE/eKY9RztQ=
 github.com/pkg/diff v0.0.0-20210226163009-20ebb0f2a09e/go.mod h1:pJLUxLENpZxwdsKMEsNbx1VGcRFpLqf3715MtcvvzbA=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
@@ -24,13 +24,13 @@ github.com/sirupsen/logrus v1.9.3 h1:dueUQJ1C2q9oE3F7wvmSGAaVtTmUizReu6fjN8uqzbQ
 github.com/sirupsen/logrus v1.9.3/go.mod h1:naHLuLoDiP4jHNo9R0sCBMtWGeIprob74mVsIT4qYEQ=
 github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
 github.com/stretchr/testify v1.7.0/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
-github.com/stretchr/testify v1.8.4 h1:CcVxjf3Q8PM0mHUKJCdn+eZZtm5yQwehR5yeSVQQcUk=
+github.com/stretchr/testify v1.10.0 h1:Xv5erBjTwe/5IxqUQTdXv5kgmIvbHo3QQyRwhJsOfJA=
-github.com/stretchr/testify v1.8.4/go.mod h1:sz/lmYIOXD/1dqDmKjjqLyZ2RngseejIcXlSw2iwfAo=
+github.com/stretchr/testify v1.10.0/go.mod h1:r2ic/lqez/lEtzL7wO/rwa5dbSLXVDPFyf8C91i36aY=
-golang.org/x/crypto v0.10.0 h1:LKqV2xt9+kDzSTfOhx4FrkEBcMrAgHSYgzywV9zcGmM=
+golang.org/x/crypto v0.38.0 h1:jt+WWG8IZlBnVbomuhg2Mdq0+BBQaHbtqHEFEigjUV8=
-golang.org/x/crypto v0.10.0/go.mod h1:o4eNf7Ede1fv+hwOwZsTHl9EsPFO6q6ZvYR8vYfY45I=
+golang.org/x/crypto v0.38.0/go.mod h1:MvrbAqul58NNYPKnOra203SB9vpuZW0e+RRZV+Ggqjw=
 golang.org/x/sys v0.0.0-20220715151400-c0bba94af5f8/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
-golang.org/x/sys v0.9.0 h1:KS/R3tvhPqvJvwcKfnBHJwwthS11LRhmM5D59eEXa0s=
+golang.org/x/sys v0.33.0 h1:q3i8TbbEz+JRD9ywIRlyRAQbM0qF7hu24q3teo2hbuw=
-golang.org/x/sys v0.9.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.33.0/go.mod h1:BJP2sWEmIv4KK5OTEluFJCKSidICx8ciO85XgH3Ak8k=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/check.v1 v1.0.0-20201130134442-10cb98267c6c h1:Hei/4ADfdWqJk1ZMxUNpqntNwaWcugrBjAiHlqqRiVk=
 gopkg.in/check.v1 v1.0.0-20201130134442-10cb98267c6c/go.mod h1:JHkPIbrfpd72SG/EVd6muEfDQjcINNoR0C8j2r3qZ4Q=

main.go

@@ -164,9 +164,21 @@ func mailHandler(peer smtpd.Peer, env smtpd.Envelope) error {
 		"uuid": generateUUID(),
 	})
 
-	if *remotesStr == "" && *command == "" {
+	var envRemotes []*Remote
+
+	if *strictSender {
+		for _, remote := range remotes {
+			if remote.Sender == env.Sender {
+				envRemotes = append(envRemotes, remote)
+			}
+		}
+	} else {
+		envRemotes = remotes
+	}
+
+	if len(envRemotes) == 0 && *command == "" {
 		logger.Warning("no remote_host or command set; discarding mail")
-		return nil
+		return smtpd.Error{Code: 554, Message: "There are no appropriate remote_host or command"}
 	}
 
 	env.AddReceivedLine(peer)
@@ -183,7 +195,7 @@ func mailHandler(peer smtpd.Peer, env smtpd.Envelope) error {
 		environ = append(environ, fmt.Sprintf("%s=%s", "SMTPRELAY_PEER", peerIP))
 
 		cmd := exec.Cmd{
-			Env: environ,
+			Env:  environ,
 			Path: *command,
 		}
 
@@ -200,7 +212,7 @@ func mailHandler(peer smtpd.Peer, env smtpd.Envelope) error {
 		cmdLogger.Info("pipe command successful: " + stdout.String())
 	}
 
-	for _, remote := range remotes {
+	for _, remote := range envRemotes {
 		logger = logger.WithField("host", remote.Addr)
 		logger.Info("delivering mail from peer using smarthost")
 
@@ -222,7 +234,7 @@ func mailHandler(peer smtpd.Peer, env smtpd.Envelope) error {
 					"err_msg":  err.Msg,
 				}).Error("delivery failed")
 			default:
-				smtpError = smtpd.Error{Code: 554, Message: "Forwarding failed"}
+				smtpError = smtpd.Error{Code: 421, Message: "Forwarding failed"}
 
 				logger.WithError(err).
 					Error("delivery failed")

smtp.go

@@ -49,7 +49,7 @@ type Client struct {
 	helloError error  // the error from the hello
 }
 
-// Dial returns a new Client connected to an SMTP server at addr.
+// Dial returns a new [Client] connected to an SMTP server at addr.
 // The addr must include a port, as in "mail.example.com:smtp".
 func Dial(addr string) (*Client, error) {
 	conn, err := net.Dial("tcp", addr)
@@ -60,7 +60,7 @@ func Dial(addr string) (*Client, error) {
 	return NewClient(conn, host)
 }
 
-// NewClient returns a new Client using an existing connection and host as a
+// NewClient returns a new [Client] using an existing connection and host as a
 // server name to be used when authenticating.
 func NewClient(conn net.Conn, host string) (*Client, error) {
 	text := textproto.NewConn(conn)
@@ -167,7 +167,7 @@ func (c *Client) StartTLS(config *tls.Config) error {
 }
 
 // TLSConnectionState returns the client's TLS connection state.
-// The return values are their zero values if StartTLS did
+// The return values are their zero values if [Client.StartTLS] did
 // not succeed.
 func (c *Client) TLSConnectionState() (state tls.ConnectionState, ok bool) {
 	tc, ok := c.conn.(*tls.Conn)
@@ -207,7 +207,7 @@ func (c *Client) Auth(a smtp.Auth) error {
 	}
 	resp64 := make([]byte, encoding.EncodedLen(len(resp)))
 	encoding.Encode(resp64, resp)
-	code, msg64, err := c.cmd(0, strings.TrimSpace(fmt.Sprintf("AUTH %s %s", mech, resp64)))
+	code, msg64, err := c.cmd(0, "%s", strings.TrimSpace(fmt.Sprintf("AUTH %s %s", mech, resp64)))
 	for err == nil {
 		var msg []byte
 		switch code {
@@ -233,7 +233,7 @@ func (c *Client) Auth(a smtp.Auth) error {
 		}
 		resp64 = make([]byte, encoding.EncodedLen(len(resp)))
 		encoding.Encode(resp64, resp)
-		code, msg64, err = c.cmd(0, string(resp64))
+		code, msg64, err = c.cmd(0, "%s", resp64)
 	}
 	return err
 }
@@ -242,7 +242,7 @@ func (c *Client) Auth(a smtp.Auth) error {
 // If the server supports the 8BITMIME extension, Mail adds the BODY=8BITMIME
 // parameter. If the server supports the SMTPUTF8 extension, Mail adds the
 // SMTPUTF8 parameter.
-// This initiates a mail transaction and is followed by one or more Rcpt calls.
+// This initiates a mail transaction and is followed by one or more [Client.Rcpt] calls.
 func (c *Client) Mail(from string) error {
 	if err := validateLine(from); err != nil {
 		return err
@@ -264,8 +264,8 @@ func (c *Client) Mail(from string) error {
 }
 
 // Rcpt issues a RCPT command to the server using the provided email address.
-// A call to Rcpt must be preceded by a call to Mail and may be followed by
+// A call to Rcpt must be preceded by a call to [Client.Mail] and may be followed by
-// a Data call or another Rcpt call.
+// a [Client.Data] call or another Rcpt call.
 func (c *Client) Rcpt(to string) error {
 	if err := validateLine(to); err != nil {
 		return err
@@ -288,7 +288,7 @@ func (d *dataCloser) Close() error {
 // Data issues a DATA command to the server and returns a writer that
 // can be used to write the mail headers and body. The caller should
 // close the writer before calling any more methods on c. A call to
-// Data must be preceded by one or more calls to Rcpt.
+// Data must be preceded by one or more calls to [Client.Rcpt].
 func (c *Client) Data() (io.WriteCloser, error) {
 	_, _, err := c.cmd(354, "DATA")
 	if err != nil {
@@ -445,9 +445,7 @@ func (c *Client) Noop() error {
 
 // Quit sends the QUIT command and closes the connection to the server.
 func (c *Client) Quit() error {
-	if err := c.hello(); err != nil {
+	c.hello() // ignore error; we're quitting anyhow
-		return err
-	}
 	_, _, err := c.cmd(221, "QUIT")
 	if err != nil {
 		return err
@@ -455,7 +453,7 @@ func (c *Client) Quit() error {
 	return c.Text.Close()
 }
 
-// validateLine checks to see if a line has CR or LF as per RFC 5321
+// validateLine checks to see if a line has CR or LF as per RFC 5321.
 func validateLine(line string) error {
 	if strings.ContainsAny(line, "\n\r") {
 		return errors.New("smtp: A line must not contain CR or LF")

smtprelay.ini

@@ -34,6 +34,10 @@
 ; accepting mails from client.
 ;local_forcetls = false
 
+; Only use remotes where FROM EMail address in received
+; EMail matches remote_sender.
+;strict_sender = false
+
 ; Socket timeout for read operations
 ; Duration string as sequence of decimal numbers,
 ; each with optional fraction and a unit suffix.
@@ -79,7 +83,7 @@
 ; authentication before they can send mail.
 ; File format: username bcrypt-hash [email[,email[,...]]]
 ;   username: The SMTP auth username
-;   bcrypt-hash: The bcrypt hash of the pasword (generate with "./hasher password")
+;   bcrypt-hash: The bcrypt hash of the pasword
 ;   email: Comma-separated list of allowed "from" addresses:
 ;          - If omitted, user can send from any address
 ;          - If @domain.com is given, user can send from any address @domain.com