Implement multiple listeners and improve documentation in ini file

smtp-proxy.ini

@@ -1,15 +1,40 @@
 ; smtp-proxy configuration
 
+; Hostname for this SMTP server
 ;hostname = "localhost.localdomain"
+
+; Welcome message for clients
 ;welcome_msg = "<hostname> ESMTP ready."
 
-;local_host = localhost
-;local_port = 25
-;local_cert =
-;local_key  =
+; Listen on the following addresses for incoming
+; unencrypted connections.
+;listen = 127.0.0.1:25 [::1]:25
+
+; STARTTLS and TLS are also supported but need a
+; SSL certificate and key.
+;listen = tls://127.0.0.1:465 tls://[::1]:465
+;listen = starttls://127.0.0.1:587 starttls://[::1]:587
+;local_cert = smtpd.pem
+;local_key  = smtpd.key
+
+; Enforce encrypted connection on STARTTLS ports before
+; accepting mails from client.
 ;local_forcetls = false
 
+; Relay all mails to this SMTP server
+
+; GMail
 ;remote_host = smtp.gmail.com
 ;remote_port = 587
+
+; Mailgun.org
+;remote_host = smtp.mailgun.org
+;remote_port = 587
+
+; Mailjet.com
+;remote_host = in-v3.mailjet.com
+;remote_port = 465
+
+; Authentication credentials on outgoing SMTP server
 ;remote_user =
 ;remote_pass =