Microsoft IP addresses landed on blacklists (Jan. 2023)

Mail[German]Brief note to the my readers, perhaps someone has observed something similar with customers or within its enterprise environments. It seems that some IP addresses that can be assigned to Microsoft mail servers have ended up on blacklists. It may then no longer be possible to send mails from these mail servers, as they are rejected by the recipient during a check.


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A German blog reader informed me about this a couple of days ago in a private message on Facebook (thanks for that). The reader writes that he came across the fact that some Microsoft IP addresses are on blacklists and are therefore rejected when sending emails. His customer's IP address was 40.107.20.44.

I just checked it on mxtoolbox.com, but at that time the IP address seems to have been removed from these lists again. The blog reader said that it may have affected more customers. According to the IP details above, this is probably a European relay, which is causing issues here. The German relay servers were probably not affected on that day.

A test email that the reader had sent via a German customer tenant went through to the recipient without any problems. As the reader wrote: "But as my customers also receive many emails via the European servers, this is really annoying." Later the reader informed me, that that the provider NoSpam-Proxy has also published a German post on the subject with information on increased error detection due to real-time blocklists.

My German article is working quite well as a honeypot – the user comments show that it is probably not an isolated case. Has anyone had similar experiences?


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