550 5.7.708 error: traffic not accepted from this IP
Microsoft's 550 5.7.708 means it banned sending from your tenant because most of its traffic looked suspicious. Fix the cause, then contact Microsoft support.
What 550 5.7.708 means
550 5.7.708 Access denied, traffic not accepted from this IPListed by Microsoft alongside 5.7.705 (tenant has exceeded threshold): most of the tenant's mail looked suspicious, so its sending is banned.
List problem or sender problem?
Sender problem. Microsoft judged most of your tenant's mail as suspicious and stopped it sending. It's a permanent failure: the server won't accept this message if you send it again unchanged.
How to fix 550 5.7.708
- Find and fix the cause first: a compromised account, an open relay, or a burst of unwanted mail.
- Then contact Microsoft support through the normal channel, which is Microsoft's stated next step.
In SendingOS, 550 5.7.708 shows on the conversation with its reason and code. If bounces like this pile up on one domain, a domain health rule can pause that domain's inboxes before the whole campaign suffers.
Not sure whether a spike in bounces is the list or the domain? Read High email bounce rate? Burned domain or bad list.
Related codes
Sources: Microsoft: NDRs in Exchange Online