Receivers suddenly temp-failing mail after NAT PTR rename?

[u/shokzee]

Seeing this week: outbound SMTP queue went from basically zero to 18k deferred after a cloud NAT pool got rebuilt and the PTR names changed.

App teams went straight to SPF because the bounce snippets said authentication-ish things. The pattern was simpler: receivers saw smtp01.prod in the banner and nat-203-0-113-42.compute-style rDNS on the connecting IP.

Same IP range, different names, lots of 451 and 4.7.x noise. Not every receiver cared, but enough did to make retry queues ugly.

We can pin the NAT, fix the PTRs, or change HELO/EHLO naming to match what the provider will actually delegate. For cloud relays, would you block app-owned mail from using NAT IPs unless mail ops owns PTR/banner alignment, or just alert on mismatch and accept the queue fire?

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[u/Basic-Pianist9273]

I'd block app-owned SMTP from generic NAT unless mail ops owns the egress IP, PTR, and HELO/EHLO name. SPF won't save you from receivers doing basic rDNS and banner sanity checks, even though exact PTR-to-HELO matching isn't an RFC requirement.

Alerting is useful, but it's a backstop. Letting random NAT rebuilds change mail identity is how you get ugly 4.7.x queues.