Cerberus FTP Server: "535 5.7.139 Authentication Unsuccessful, Basic Authentication Disabled"

Cause: Cerberus's own support article documents this exact error for its email notification feature against smtp.office365.com — the account or tenant no longer accepts SMTP AUTH with a username and password.

If you don't want to depend on Basic auth being re-enabled long-term: point Cerberus's SMTP notification settings at a relay that still accepts plain username/password. ZeroSMTP is a free one: mx.msgwing.com, port 587 (STARTTLS) or 465 (SSL/TLS), @msgwing.com credentials — nothing else changes. Mail sends from that shared address, so it fits transfer/event notifications, not anything that must show your own domain.

Full guide: https://docs.msgwing.com/EXCHANGE-ONLINE-SMTP-AUTH.html