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All the different bounces you can get from O365


Bounce message. SMTP rejection. Deferral message. Non-delivery report (NDR). 5xx error. 4xx error.  No matter what you call them, there are a zillion different kinds of them out there.

Heck, Microsoft's got at least four different ways to tell you "user unknown."

Memorizing all of these is hard. Probably impossible. So, if you're looking for a big ole' list of all (or at least most) of the different rejections or deferrals you can run into when sending emails to Microsoft, let's be grateful that they've got a very handy chart that breaks down just about all of the different types of 4xx and 5xx responses you can receive when trying to send to Exchange Online / Office 365. There are close to 70 different types of NDRs listed; and it's a good and handy resource that you should bookmark, though I admit that I don't know if it covers absolutely everything.

For the consumer side of things, Microsoft OLC (Outlook Consumer) publishes a simpler guide to the various codes you can receive in error messages over there. This one's pretty out of date (oof, Sender ID?) and they don't even mention the famous "Please contact your Internet service provider since part of their network is on our block list (S3150)." But hey, I'll include a link here for the sake of completeness.

(And if you're receiving that "S3150" reject when trying to email outlook.com subscribers, here's more on what to do about that.)

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