Deferrals at Microsoft

If you’re seeing a lot of “451 4.7.500 Server busy. Please try again later” from Office365 this morning you’re not alone.

Microsoft are aware of the issue, and incident EX680695 says:

Current status: We’ve identified that specific IP addresses are being unexpectedly limited by our anti-spam procedures, causing inbound external email delivery to become throttled and delayed. We’re reviewing if there have been any recent changes to our anti-spam rules to understand why the IP addresses are being limited. In the meantime, we’re manually adding reported affected IP addresses to an allowed list to provide immediate relief.

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A c/p from an email I sent to a mailing list.
I think we’re seeing a new normal, or are still on the pathway to a new normal. Here’s my theory.
1) Hotmail made a lot of underlying code changes, learning from 2 decades of spam filtering. They had a chance to write a new codebase and they took it.
2) The changes had some interesting effects that they couldn’t test for and didn’t expect. They spent a month or two shaking out the effects and learning how to really use the new code.
3) They spent a month or two monitoring. Just watching. How are their users reacting? How are senders reacting? How are the systems handling everything?
3a) They also snagged test data along the way and started learning how their new code base worked and what it can do.
4) As they learned more about the code base they realized they can do different and much more sophisticated filtering.
5) The differences mean that some mail that was previously OK and making it to the inbox isn’t any longer.
5a) From Microsoft’s perspective, this is a feature not a bug. Some mail that was making it to the inbox previously isn’t mail MS thinks users want in their inbox. So they’re filtering it to bulk. I’ll also step out on a limb and say that most of the recipients aren’t noticing or caring about the missing mail, so MS sees no reason to make changes to the filters.
6) Expect at least another few rounds of tweak and monitor before things settle into something that changes more gradually.
Overall, I think delivery at Microsoft really is more difficult and given some of the statements coming out of MS (and some of the pointed silence) I don’t think they’re unhappy with this.

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