Allow config to have multiple remotes.

This will enable development teams to test emails from
a system like mailcatcher while also having the email
delivered to the indented mailbox.

go.mod

@@ -4,6 +4,7 @@ require (
 	github.com/chrj/smtpd v0.3.1
 	github.com/google/uuid v1.3.0
 	github.com/sirupsen/logrus v1.8.1
+	github.com/stretchr/testify v1.7.0
 	github.com/vharitonsky/iniflags v0.0.0-20180513140207-a33cd0b5f3de
 	golang.org/x/crypto v0.0.0-20201221181555-eec23a3978ad
 )