Emails with attachments being bounced back as spam

Every email I send containing ANY type of attachment - Word, PDF, Zip, etc - to ANY recipient is being bounced back unread as spam (code 550).

This happen's whether I send it from iCloud, my Macbook Air or my iPhone.

It happens whether the recipient's email address is personal, private or a company's.

It started happening this week. I have updated to Sequioa OS since and there has been no change.