What the message looks like
550 5.7.606 Access denied, banned sending IP [192.0.2.1]550 5.7.708 Access denied, traffic not accepted from this IP550 5.7.511 Access denied, banned sender550 5.7.501 Access denied, spam abuse detected
What it means
Microsoft's servers, used by Outlook.com, Hotmail and Microsoft 365, refused the email because they've blocked the sending server or the sender. The block isn't about the recipient, so every Microsoft address rejects it in the same way.
Microsoft uses a range of related codes:
5.7.606to5.7.649- the sending IP address is on Microsoft's own blocklist.5.7.708- Microsoft doesn't accept mail from that IP at all, usually because it has a poor reputation or belongs to a new or trial Microsoft 365 tenant.5.7.501,5.7.511and other "banned sender" codes - the sender's address or domain has been blocked for spam.
If you're the recipient
There's nothing to change on your side, as you can't allowlist your way past these blocks. Let the sender know, and pass on the full error message if you can.
If you're the sender
For 5.7.606 to 5.7.649, you can ask Microsoft to remove your IP address with its delist portal at sender.office.com, once you've fixed whatever caused the spam. 5.7.708 usually can't be delisted that way, so send through an established email provider, or ask your current provider for a different IP. For banned sender codes, contact Microsoft support.
If you use addy.io
If your Outlook or Microsoft 365 inbox rejects an email that addy.io forwards with one of these codes, it appears on your Failed Deliveries page, and the block is on addy.io's sending IP. Please get in touch with the full error, and consider adding a different recipient in the meantime.