endpoint/smtp: Allow to change the line length limit enforced by go-smtp

See #352.

Author: foxcpp

docs/man/maddy-smtp.5.scd

@@ -21,10 +21,11 @@ smtp tcp://0.0.0.0:25 {
     auth pam
     defer_sender_reject yes
     dmarc yes
-	limits {
-		endpoint rate 10
-		endpoint concurrency 500
-	}
+    smtp_max_line_length 4000
+    limits {
+        endpoint rate 10
+        endpoint concurrency 500
+    }
 
     # Example pipeline ocnfiguration.
     destination example.org {
@@ -139,6 +140,19 @@ Store message bodies smaller than _max_size_ entirely in RAM, otherwise write
 them out to the FS.
 _path_ can be omitted and defaults to StateDirectory/buffer.
 
+*Syntax*: smtp_max_line_length _integer_ ++
+*Default*: 4000
+
+The maximum line length allowed in the SMTP input stream. If client sends a
+longer line - connection will be closed and message (if any) will be rejected
+with a permanent error.
+
+RFC 5321 has the recommended limit of 998 bytes. Servers are not required
+to handle longer lines correctly but some senders may produce them.
+
+Unless BDAT extension is used by the sender, this limitation also applies to
+the message body.
+
 *Syntax*: dmarc _boolean_ ++
 *Default*: yes
 

internal/endpoint/smtp/smtp.go

@@ -256,6 +256,7 @@ func (endp *Endpoint) setConfig(cfg *config.Map) error {
 	}, bufferModeDirective, &endp.buffer)
 	cfg.Custom("tls", true, endp.name != "lmtp", nil, tls2.TLSDirective, &endp.serv.TLSConfig)
 	cfg.Bool("insecure_auth", endp.name == "lmtp", false, &endp.serv.AllowInsecureAuth)
+	cfg.Int("smtp_max_line_length", false, false, 4000, &endp.serv.MaxLineLength)
 	cfg.Bool("io_debug", false, false, &ioDebug)
 	cfg.Bool("debug", true, false, &endp.Log.Debug)
 	cfg.Bool("defer_sender_reject", false, true, &endp.deferServerReject)