Hi @Fridolin Metzger ,
I wonder how you send the emails from your website to office 365?
Are you logged in to a specific email address? Or is it sending emails through built-in code in the background?
Besides, here is a general solution to this error ,you could refer to the solution to create a relay connector to see if IP throttling has stopped.
Fix email delivery issues for error code 451 4.7.500-699 (ASxxx) in Exchange Online | Microsoft Learn
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