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550 5.7.511 error: banned sender (Microsoft)

Microsoft's 550 5.7.511 means the IP you're sending from is banned. Fix what got it banned, then ask for delisting at [email protected].

Sender problemYou are the problem: your domain, IP, authentication or content. Fix that before sending more.

What 550 5.7.511 means

Microsoft 365 / Outlook550 5.7.511 Access denied, banned sender

The IP you're sending from was banned by Microsoft.

List problem or sender problem?

Sender problem. It's your sending IP: Microsoft has it on its ban list. It's a permanent failure: the server won't accept this message if you send it again unchanged.

How to fix 550 5.7.511

  1. Fix the cause first: compromised accounts, complaints, bad lists. An IP that's delisted and keeps sending the same way gets banned again.
  2. Email [email protected] with the full bounce and the IP address, or use Microsoft's delist portal.
  3. On a shared sending IP, tell your provider: the ban may not be about your mail.

In SendingOS, 550 5.7.511 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.

550 5.7.708550 5.7.1451 4.7.500

Sources: Microsoft: NDRs in Exchange Online