Bouncing email due to bad initial MX TTL?

Joshua Marshall asked:

Last Friday night we changed mail servers. We moved off Office 365 to another hosted Exchange Solution (

We use Godaddy for DNS, and after the server migration stuff was ready and clients were good to go, I edited our MX records. Godaddy is fast so within an hour or so I saw on that the new, proper MX records had been propagating nicely.

Queue Monday morning. Email is coming through but I’m starting to hear about bouncebacks. More of the same Tuesday. I’d been pulling my hair out when I just eyeball our DNS entries on Tuesday night and see that the TTL on the new MX record is 1 Week. Yikes. I change it to 1/2 an hour. Today, one bottleneck messagelabs/symantec) has updated to point to the right server, but we’re still getting some unfortunately large external senders, ie Postini, bouncing their messages off the old server.

Is that initial 1 week TTL to blame? Would Postini resepct that initial TTL despite my haviong shortened it yesterday? I’m going nuts because it seems pretty helpless. I had IT folks at one of our clients contact Postini to open a ticket, since their user emails to us are bouncing, but it could take time to move on that front. My only hope is that it’s Thanksgiving so work is mostly over until the weekend. Friday night will be the official 1 week from that initial bad TTL. Should I have hope that things will ‘just work’ come Monday? I don’t know what else to check. The domain is removed from O365, the new MX records seem nicely propagated. I’m about to jump off a pier.

My answer:

Your scenario is most plausible.

Remember that DNS does not “propagate”; rather, records are cached by other DNS servers for the duration of the TTL. So some sites may be caching that old record for as long as a week. (And broken DNS servers may cache it even longer, but those are fortunately few and far between.)

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