Exchange Online now blocking whitelisted domain

DThimsen

From what you've written I think you are in the wrong place. If you look at my post, while I'm talking about the anti-spam policy page, I was not talking about editing the Anti-spam inbound policy" !

Here again with additional clarification where that is:

Email & collaboration ⇒ Policies & Rules ⇒ Threat policies ⇒ Policies: Anti-spam ⇒ click "Connection filter policy (Default)" ⇒ Add IPs or IP Ranges in CIDR format in "IP Allow list"

In my screenshot there are way more IPs allow-listed than I need but I can't be bothered to clean up the list until things work again. Since my change was done no additional mails were rejected but I can't be sure since my change was done roughly at the end of the business day and mail volume in the evening is lower than during the day.