Add new field Banner in Server struct for custom smtp greeting text

[iredmail]

• Add new field Banner in Server struct for custom smtp greeting text.
• Keep Domain field for backward compatible, but it's nowhere used. (It was only used in greeting text, which is now replaced by Banner)
• Use better text for QUIT command (Goodnight and good luck -> Bye)

See also #131 (item 2)

[iredmail]

Dear @emersion

Could you help review and let me know your opinion? Thanks. :)

[emersion]

Per the RFC, the domain name in the greeting is mandatory:

   Greeting       = ( "220 " (Domain / address-literal)
                  [ SP textstring ] CRLF ) /
                  ( "220-" (Domain / address-literal)
                  [ SP textstring ] CRLF
                  *( "220-" [ textstring ] CRLF )
                  "220" [ SP textstring ] CRLF )

Other than that, I'm not sure what purpose a custom greeting would have, so I'm not convinced this change is worth merging.

[iredmail]

Dear @emersion,

Thanks for the reply.

• I will update PR and keep the domain name in greeting text.
• The custom greeting text is some kind of branding thing. :)

[emersion]

The custom greeting text is some kind of branding thing. :)

This doesn't sound very convincing to me.

[iredmail]

OK. i will close this PR.
But how about change few words in SMTP reply message?

• Speak up -> Invalid command. https://github.com/emersion/go-smtp/blob/master/conn.go#L106
• Goodnight and good luck -> Bye (used by Postfix). https://github.com/emersion/go-smtp/blob/master/conn.go#L143

[emersion]

Sure, fine by me. Doing the same as Postfix sounds like a good idea.

[iredmail]

Sure, fine by me. Doing the same as Postfix sounds like a good idea.

PR #133 passed CI and waiting for approval. :)