Discourse SMTP sends "EHLO localhost" instead of domain, breaking Google smtp-relay

[Syonyk (Russell Graves)]

@Falco

The thing here is that it is working for all the tens of thousands of Discourse instances that don’t rely on Google for email at the moment. Changing a default like that can break everyone while fixing for the, comparatively small, Google Apps users.

Yes, but sending ‘localhost’ (to a remote host) is also wrong, by RFC.

RFC 5321: Simple Mail Transfer Protocol

Emphasis mine.

4.1.1.1 Extended HELLO (EHLO) or HELLO (HELO)
These commands are used to identify the SMTP client to the SMTP
server. The argument clause contains the fully-qualified domain name
of the SMTP client, if one is available. In situations in which the
SMTP client system does not have a meaningful domain name (e.g., when
its address is dynamically allocated and no reverse mapping record is
available), the client SHOULD send an address literal (see
Section 4.1.3).

Older RFCs say that the server should not reject clients based on the EHLO string, which Google seems to be doing, but I don’t see that phrasing in 5321.

@pfaffman

OK. I’ll agree that it probably makes sense to err on the side of caution, but my guess is that any mail server that will accept localhost would also accept DISCOURSE_HOSTNAME , but I don’t have any, like, data.

I would expect any remote mailserver that tolerates localhost to tolerate (and prefer) a FQDN as required by RFC. I understand the desire not to break things, but as I read the relevant RFCs, Discourse is simply wrong by default, and that it works is a result of excessively permissive remote SMTP servers.