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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Microsoft changes

There’s been quite a bit of breakage and delivery failure to various Microsoft domains this month. It started with them changing the MX for hotmail.co.uk, then the MX for hotmail.fr… and both these things seem to have broken mail. I also saw a report this morning that some of the new MXs have TLS certificates that don’t match the hostnames.

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Microsoft using Spamhaus Lists

An on the ball reader sent me a note today showing a bounce message indicating microsoft was rejecting mail due to a Spamhaus Blocklist Listing.
5.7.1 Client host [10.10.10.10] blocked using Spamhaus. To request removal from this list see http://www.spamhaus.org/lookup.lasso (S3130). [VE1EUR03FT043.eop-EUR03.prod.protection.outlook.com]
The IP in question is listed on the CSS, which means at a minimum Microsoft is using the SBL. I expect they’re actually using the ZEN list. ZEN provides a single lookup for 3 different lists: the SBL, XBL and PBL. The XBL is a list of virus infected machines and the PBL is a list of IPs that the IP owners state shouldn’t be sending email. Both of these lists are generally safe to use. If MS is using the SBL, it’s very likely they’re using the other two as well.
 

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Don’t break the (RFC) rules

It looks like Microsoft are getting pickier about email address syntax, rejecting mail that uses illegal address formats. That might be what’s causing that “550 5.6.0 CAT.InvalidContent.Exception: DataSourceOperationException, proxyAddress: prefix not supported – ; cannot handle content of message” rejection.

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