Microsoft 550 DY-001: mail from dynamic address space
Microsoft / Outlookpolicy_blockaction: fix_config
550 DY-001 means Outlook.com will not accept mail directly from an address that appears to be in dynamic or residential space. It is not about your message or your reputation; the address itself is the problem.
This is the one Microsoft code that no amount of warming fixes. The classification follows the address range, so the answer is different infrastructure rather than better behaviour.
What it looks like in the log
550 DY-001 Mail rejected by Outlook.com for policy reasonsWhy it happens
- Sending directly from a home or office connection.
- A provider whose address range is classified as dynamic regardless of how it is actually assigned.
- No reverse DNS, or reverse DNS that still carries the provider's generic naming.
What to do
- Send through a relay on address space intended for mail, or move to a provider whose ranges are not classified this way.
- Set a reverse DNS record that matches the name used in EHLO. A generic provider-assigned name reads as dynamic even when it is not.
- Check whether the address is also on the policy lists that record dynamic space, because those follow the same classification.
Related
Blocklist check checks the policy lists that record dynamic space, which classify an address the same way Microsoft just did. Open blocklist check