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Error code wiki · 19 codes

Mimecast SMTP error codes

Mimecast runs a cloud email security gateway that many companies point their MX records at, so its servers accept or refuse mail on behalf of those companies.

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Permanent failures (5xx)

13 codes

The receiving server refused the message for good. Your server will not try again, and the message comes back to you as a bounce.

  1. 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.Permanent
  2. 550Blocked by a sender policy or a failed SPF check.The recipient company’s Mimecast account refused your sender address or domain, either through a Blocked Senders policy or because the message hard-failed SPF.Permanent
  3. 550Rejected by an anti-spoofing policy.The message claims to come from one of the recipient company’s own domains but arrived from outside, so Mimecast’s anti-spoofing policy rejected it.Permanent
  4. 550The recipient blocked you personally.A person at the recipient company, or their administrator, added your address or domain to their own blocked senders list in Mimecast, so your mail is refused.Permanent
  5. 550Blocked by the company’s Blocked Senders policy.An administrator of the recipient’s Mimecast account set a Blocked Senders policy that matches your From or envelope sender address, so the message is refused.Permanent
  6. 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.Permanent
  7. 550SPF check failed.Mimecast rejected the message because the IP that sent it is not listed in the SPF record of the sender’s domain, and the recipient’s policy rejects SPF failures.Permanent
  8. 550DMARC check failed.Mimecast rejected the message on DMARC grounds: the authentication it found did not satisfy the DMARC policy of your From domain.Permanent
  9. 550Blocked by a content rule.Something in your message matched a content rule the recipient company set in Mimecast, such as a word, phrase or attachment type, and the message was refused.Permanent
  10. 554Rejected as spam or a virus.Mimecast’s scanning found a virus signature or gave the message a spam score above the maximum, so the recipient’s gateway refused it.Permanent
  11. 554Rejected for a malicious QR code.Mimecast read a QR code in your message, followed the link inside it and judged that link malicious, so the message was rejected.Permanent
  12. 554Message too large.Your message is larger than the recipient company’s size limit in Mimecast, or larger than Mimecast’s own limit, so it was refused.Permanent
  13. 554Blocked by country of origin.The recipient company blocks mail from the country your sending IP is located in, using a Mimecast Geographical Restrictions policy.Permanent

Temporary failures (4xx)

6 codes

The receiving server cannot take the message now. Your server keeps it and retries, and only bounces it if the retries run out.

  1. 421Sending IP blocked by a Blocked Senders policy.The recipient company’s Mimecast account has a Blocked Senders policy covering your sending IP, so Mimecast refuses the connection until that entry is removed.Temporary
  2. 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.Temporary
  3. 451Mimecast server at full load.The Mimecast server your message reached was at maximum load and asked your server to try again later. Nothing is wrong with your mail.Temporary
  4. 451Mimecast does not recognise either domain.Mimecast saw a message where neither the sender’s nor the recipient’s domain belongs to its customer, so it refused to pass it on, as relaying for outsiders is not allowed.Temporary
  5. 451Sending IP temporarily blocked for exceeding a limit.Your sending IP went over a limit Mimecast applies, so it is refused for now. Mail is accepted again once your traffic falls back under the limit.Temporary
  6. 452More than 100 recipients in one message.Mimecast accepts at most 100 recipients per message transaction and asks your server to send the rest in another batch. Most mail servers do this automatically.Temporary

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