550 5.7.708 is a permanent SMTP reply meaning microsoft 365: traffic not accepted from this ip. Microsoft found that most of the mail from your tenant looks suspicious, so it banned outbound mail from it. This is a block on your tenant, not on one message.
What 550 5.7.708 means
Microsoft found that most of the mail from your tenant looks suspicious, so it banned outbound mail from it. This is a block on your tenant, not on one message.
Common causes
A mailbox in the tenant was compromised
Most commonA stolen password let a spammer send large volumes through your tenant.
A new tenant sent a lot of mail at once
CommonNew Microsoft 365 tenants have low trust. Bulk or cold email from them is quickly limited.
An open relay or connector is abused
Less commonAn inbound connector or app relay lets outside senders send through you.
How to fix 550 5.7.708
- 1
Stop the abuse
Reset passwords, turn on MFA, remove bad inbox rules and connectors, and check the outbound spam report.
- 2
Contact Microsoft support
Microsoft says to make sure any compromise or open relay is fixed, then contact support through your normal channel to lift the block.
- 3
Do not send bulk or cold email from Microsoft 365
Use a dedicated sending service and separate domains for bulk mail, and watch those domains for problems.
Confirm your domain is set up correctly
Most delivery failures — permanent or temporary — trace back to authentication or reputation. Run a free EmailHealth check to confirm SPF, DKIM, DMARC, blacklist status, and TLS are all healthy for your sending domain. If you send in volume, free daily monitoring will alert you before a small issue becomes a flood of bounces.