Why is my Microsoft 365 outbound email being blocked as spam?

Outbound blocks in M365 usually stem from compromised accounts, bad sending IP reputations, or sending high-volume marketing mail from a basic transactional mailbox.

“This is usually a reputation issue, either a compromised account or marketing volume going out of a mailbox meant for transactional mail. I’ll pin down the cause and have it resolved within 24 hours for $99.”
— Jeff Bollettino, Managing Director, Ignite Conversions