“message couldn’t be delivered because you weren’t recognized as a valid sender” error

In this situation, I suggest that you use an admin account to log into Exchange Admin Center, and then click Protection> Action center to check if your account is blocked.
Here is an article for your reference:

Removing a user, domain, or IP address from a block list after sending spam email.

Note: This needs to be completed by a global admin in your Office 365 organization. Please let your admins do this if you aren’t one.

If the issue persists after unblocking your Office 365 account, we need to collect the your tenant information to help you resolve it. For privacy, I have sent you a private message to collect them.

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