Starting a few weeks back, many recipients have found my email coming from Thunderbird was going to their spam folder. The got a report that their email system cannot verify that my mail came from wisc.edu. My user name on the University's outlook.office365.com system is [email protected]. This happens with different recipients using different email clients. Some are individuals, some were corporations. I found one clue online, that I should enable MFA, a good idea anyway, so I have done that. But a recipient (using the client at the State University of New York - Fredonia) has now reported my email going to spam again. My email setup has the address [email protected], going through IMAP server outlook.office365.com, using connection security STARTTLS, with authentication method OAuth2. It all worked well for years, perhaps until the update to Thunderbird 78.9.1. Since people don't check their spam folder looking for incoming email, I cannot be exactly sure when it started. And since several of the people reporting the problem have (set by some authority, they can't change it) their spam folder emptied after a week or so, we can't look back to find the first instance. Thanks for any help!