Update 16:06 GMT +2
Answer to my ticket:
Please note we have run a backend diagnostic related to the issue, and it will take some time to come into effect. So, we would request to kindly check and confirm the behavior.
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We're experiencing the same issue in Greece. I opened a support ticket this morning, but there's no solution yet. Changing our server's IP allows us to send emails to Exchange Online temporarily. We're informing our customers that this problem is not on our end but rather with Microsoft.