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I am setting up a mail server which has separate IP addresses for sending and receiving mail. My config looks like this:

# The addresses we send out on:
smtp_bind_address = 185.xx.xx.101
smtp_bind_address6 = 2001:BA8:xx:xx:xx:A1 

# The interfaces we receive on:
inet_interfaces = 127.0.0.1, 185.xx.xx.102, 2001:BA8:xx:xx:xx:A2

I think inet_interfaces configures the listening address, so is that the one to use when sending mail out via SMTP AUTH?

TommyPeanuts
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2 Answers2

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If parameter smtp_bind_address(6) wasn't empty then value in parameter inet_interfaces doesn't affect outgoing email. Parameter inet_interfaces is consulted if inet_interfaces specifies just one IPv4 and/or IPv6 address that is not a loopback address.

In your case, email delivered via SMTP Auth will delivered via smtp_bind_address IP address.

References:

Community
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masegaloeh
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It doesn't matter, from a technical perspective. You can use any (reachable) IP address for sending your e-mail with SMTP AUTH. I assume you want to avoid getting your IP listed in a blacklist if you happen to leak any spam; that's far less likely to happen with a relay protected by SMTP AUTH; in that case, presumably, you're only sending to one (or a few) destinations, and those destinations don't have to blacklist you if you're spamming; they just disable your account and the spam stops.

womble
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  • Yes, it's blacklist paranoia, since we won't have control over the sending machines, which could get caught up in botnets. We'll be throttling the sending rate using policyd, and possibly also putting them through spamassassin. But we had a spare IP so we thought it might be useful to separate them as well perhaps. – TommyPeanuts Aug 01 '15 at 21:54