550: sending IP listed on a blocklist
Mimecast rejected the message because your sending IP appears on a public blocklist the recipient’s account checks. The rest of the reply names the list.
- Mimecast
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 550
- Provider
- Mimecast
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- Either side, depending on the cause. Start with the checks below that are yours.
What it means
Mimecast checks connecting servers against real-time blocklists (RBLs). When the sending IP is listed, the message is refused permanently and the reply carries the text of the list that matched, so the wording varies with the list. The listing belongs to the blocklist operator, not to Mimecast, which means the lasting fix is getting the IP removed from that list. Mimecast also notes that the recipient’s administrator can bypass the check for a sender with an Auto Allow or Permitted Senders policy, which helps when you need to reach one company while the delisting is pending.
Common causes
- Your sending IP, or the range it sits in, is listed on a public blocklist because of spam traffic from it.
- You share an IP with other senders whose mail got it listed.
- A compromised account or device on your network sent spam through the same IP.
How to fix it
- 01Read the rest of the reply to see which blocklist matched.
- 02Check the IP on that list, and on others, with a blacklist checker.
- 03Fix what caused the listing, then follow that list’s own removal process.
- 04If one recipient matters while you wait, ask their IT team for a Permitted Senders or Auto Allow policy for your address.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- Policies - Mimecast SMTP Error Codes (Mimecast support)mimecastsupport.zendesk.com
Related codes
- 550Mailbox unavailable.The receiving server permanently refused the message for this recipient: the mailbox does not exist, cannot be reached, or the server rejected the message by policy.
- 550Rejected for poor IP reputation.Mimecast’s ongoing reputation checks scored your sending IP as poor and rejected the message. It can follow a run of temporary 4xx deferrals.
550 at other providers
- 550 [SC-001]Outlook.com policy block for content or reputation.Outlook.com rejected the mail for policy reasons tied to spam-like content or to the reputation of the sending IP or domain.
- 550 [SC-002]Outlook.com block for namespace mining.Outlook.com blocked the sending IP because it behaved like a namespace miner, checking whether addresses exist without sending mail.
- 550 [SC-003]Outlook.com block for an open proxy or relay.Outlook.com rejected the mail because the sending IP appears to be an open proxy or open relay.
- 550 [SC-004]Outlook.com block after complaints.Outlook.com placed a block on the sending IP because it received complaints about mail coming from it.
- 550 [DY-001]Outlook.com refuses dynamic IP addresses.Outlook.com rejected the mail because the sending IP is dynamic address space, which it generally does not accept mail from.
- 550 [DY-002]Outlook.com block for a likely compromised sender.Outlook.com rejected the mail because the sending server or computer appears to be compromised or infected.
Further reading
All 19 Mimecast codesEvery code in the wikiPaste your own bounce