Predictive text is re-learning email addresses that I removed

A

AC Question

Galaxy S6 running Android 6.0.1:

When I'm at the username/password login prompt for my work email (web-based Office 365), predictive text autosuggests a few very old Gmail addresses related to my work. I've deleted them from the predictive text several times by holding down and having it un-learn the addresses, but within a week or so they inevitably pop back up and I have to repeat.

The unsettling thing is that these are very old addresses that I've literally never typed into the phone, which means that Android is scraping my Office 365 email for addresses, and is doing it repeatedly. How do I get it to stop?
 

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