Let's suppose we have an SMTP server called alpha.example.com
, which is the only MX server listed for example.com
.
Like most modern email SMTP servers, rejects emails not bound for its group of virtual users, rejects incoming email from spamhaus listed domains, runs mail through SpamAssassin, checks SPF records, all of that good stuff.
However, being the only MX record listed for example.com
, there's a concern that if the machine is taken down for maintenance, email may be delayed. Well behaved other SMTP servers will obviously queue, and retry later, but that could introduce some tens of minutes or hours of delay, even if the primary is brought down for a small amount of time if it coincides with an attempted delivery.
We want to start a secondary SMTP server, which let's call beta.example.com
, which will be listed as the lower priority MX record. But alpha.example.com
is still where all the mail is stored, it's still where all the users connect their MUAs to. So beta.example.com
will just store-and-forward email to alpha.example.com
, since it's not the "endpoint".
My question is: Does beta
need to also run its email through the same checks as alpha
? ie. checking for virtual users, spamhaus, SPF lookups, DKIM checks, or is the fact that those things are checked on alpha
when the email is delivered is sufficient
Also, will beta
relaying the emails to alpha
fall afoul of those emails SPF lookups, given that beta.example.com
is unlikely to be listed as a valid sender of arbitary.org
's emails.
Technically I'm using Postfix for SMTP (and Dovecot for the authentication/IMAP side), in case that informs the answers.