550: DMARC check failed
Mimecast rejected the message on DMARC grounds: the authentication it found did not satisfy the DMARC policy of your From domain.
- 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
- The sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
550 DMARC Sender Invalid - envelope rejected.
What it means
Mimecast evaluates DMARC for inbound mail as part of the recipient company’s DNS Authentication settings. A message passes DMARC when SPF or DKIM passes for a domain that matches the visible From domain. This reply means that did not happen and the message was refused permanently. Mimecast’s own description for this reply points to the sending IP missing from the domain’s SPF record, so that is the first thing to check, followed by whether your DKIM signature uses your From domain. Because DMARC is published by the sender’s domain, the fix is on your side.
Common causes
- The sending IP is not in the SPF record of your domain, which is the cause Mimecast gives.
- Your sending service signs DKIM with its own domain rather than yours, so neither check aligns with the From address.
- Your domain publishes a DMARC policy of reject or quarantine, and the recipient enforces it.
How to fix it
- 01Check your SPF record covers the IP that sent the message.
- 02Set up DKIM signing with your own domain in each sending service.
- 03Look up your DMARC record and its aggregate reports to see which sources fail.
- 04Resend once a test message passes DMARC.
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.
- 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.
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