Been through this many times. Have no problems and then all of a sudden, servers get blocked, same misleading error message about it being temporary, same annoying auto response that "we cannot see anything wrong" and same opaque customer services that won't tell you anything.<p>They even have the cheek to link to a 15 year old guide on email best-practices, it is in equal parts awkward, annoying and incredibly shameful for a large organisation like MS. The fact that SNDS shows all greens seems to mean nothing and clearly they have been instructed not to engage in any conversation about why it is happening and the fact these are servers that are linked to a well-known business with static IPs that have been in use for over 3 years.<p>The best I can work out is the use of heuristic mail filters that detect anomolies for whatever reason they feel like. Too much email, not enough email, email sending rates that are too erratic, basically what 99.9% of email servers in the world are doing and they decide to block you. I could live with "rate limited" and I could live with "temporary" because I would disable the servers once they got blocked but nope. Just more "we don't give a shit" attitude from Microsoft.<p>Sadly, I had to accept the inevitable which I had avoided for many years and migrate all of our email sending to Amazon SES because life is too short to send emails to Microsoft.