K-9 mail command: *sensitive*; response: #3# [NO, LOGIN failed.]

Might be related to

Also read my response on it, but in the meantime they seem to have it extended to 3rd quarter, too:

IMPORTANT: Sometime in second and third quarters of 2022 we will selectively pick tenants and disable Basic Auth for all affected protocols except SMTP AUTH for a period of 12-48 hours. After this time, Basic Auth for these protocols will be re-enabled, if the tenant admin has not already re-enabled them using our self-service tools.

Edit: also in the meantime K9 supports OAuth for office 365: