Incorrect handling of multiline errors

[pch]

I believe that multiline errors aren't being handled properly. Consider this example:

ErrRecipientDomainMxCheckFailed = &smtp.SMTPError{
	Code:         550,
	EnhancedCode: smtp.EnhancedCode{5, 1, 2},
	Message:      "Recipient domain MX check failed\nthis is another line\nand another one",
}

Returning this error will cause the following exchange:

 ~> RCPT TO:<someone@invalidemaildomain.com>
<~* 550 5.1.2 Recipient domain MX check failed
 ~> QUIT
<~  this is another line
<~  and another one
<~  221 2.0.0 Bye

This causes incorrect/premature QUIT and breaks client response readers.

Correct response should be:

<~* 550-5.1.2 Recipient domain MX check failed
<~* 550-5.1.2 this is another line
<~* 550 5.1.2 and another one

The issue seems to be here:

func (c *Conn) writeResponse(code int, enhCode EnhancedCode, text ...string) {

This function looks to be handling multiline responses correclty, but it's expecting text to be multiple arguments and it's getting a single multiline string.

A (naive) fix would be something like:

-func (c *Conn) writeResponse(code int, enhCode EnhancedCode, text ...string) {
+func (c *Conn) writeResponse(code int, enhCode EnhancedCode, text string) {
+       lines := strings.Split(text, "\n")
+
        // TODO: error handling
        if c.server.WriteTimeout != 0 {
                c.conn.SetWriteDeadline(time.Now().Add(c.server.WriteTimeout))
@@ -1236,13 +1238,13 @@ func (c *Conn) writeResponse(code int, enhCode EnhancedCode, text ...string) {
                }
        }

-       for i := 0; i < len(text)-1; i++ {
-               c.text.PrintfLine("%d-%v", code, text[i])
+       for i := 0; i < len(lines)-1; i++ {
+               c.text.PrintfLine("%d-%v", code, lines[i])
        }
        if enhCode == NoEnhancedCode {
-               c.text.PrintfLine("%d %v", code, text[len(text)-1])
+               c.text.PrintfLine("%d %v", code, lines[len(text)-1])
        } else {
-               c.text.PrintfLine("%d %v.%v.%v %v", code, enhCode[0], enhCode[1], enhCode[2], text[len(text)-1])
+               c.text.PrintfLine("%d %v.%v.%v %v", code, enhCode[0], enhCode[1], enhCode[2], lines[len(lines)-1])
        }
 }

Use case

The use case is handling gmail responses in a go-smtp server (relay mode). Here's what gmail returns:

550-5.1.1 The email account that you tried to reach does not exist. Please try
550-5.1.1 double-checking the recipient's email address for typos or
550-5.1.1 unnecessary spaces. Learn more at
550 5.1.1  https://support.google.com/mail/?p=NoSuchUser f199si8487887wmf.58 - gsmtp

Currently, it's not possible to return this error back to the client in the original form, because it's being formatted incorrectly.