550: rejected 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.
- 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 the server says
550 Local CT IP Reputation - (reject)
What it means
Mimecast keeps checking the reputation of the IPs that connect to it, and this reply means those checks have turned against the IP that sent your message. Mimecast notes it can come after earlier 4xx replies, when a sender that was being slowed down keeps showing poor signals. The rejection is permanent for that message, and the judgment is about the IP rather than about you or your domain. Mimecast’s documented options are a customer-side Auto Allow or Permitted Senders policy and a review of the source IP ranges through Mimecast.
Common causes
- Spam or spam-like traffic from your IP, such as cold mail with high complaint or bounce rates.
- A new or recently changed IP sending at volume before it has any history.
- Shared infrastructure whose other users damaged the IP’s reputation.
How to fix it
- 01Check the IP on public blocklists and look at recent complaint and bounce rates for that IP.
- 02Cut volume from the IP and stop mailing addresses that do not engage or that bounce.
- 03For recipients you need to reach now, ask their IT team for an Auto Allow or Permitted Senders policy for your address.
- 04A Mimecast customer can ask Mimecast to review the IP; give your contact the IP and the bounce text.
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.
- 451Greylisting or a reputation check delayed the message.Mimecast is holding off a first contact from your server through greylisting, or because your IP’s reputation is poor. A well-behaved server retries and usually gets through.
- 550Sending 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.
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