550: blocked 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.
- 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 recipient's side. Their mail admin has to change something; you can tell them.
- What the server says
550 Rejected by header-based Blocked Senders - Block policy for Header From.
What it means
Mimecast customers can write Blocked Senders policies that reject mail by the Header From address, by the envelope sender address, or both, and these two replies tell you which one matched. The decision was made by the receiving company’s administrators and applies across their account, so nothing in your DNS or reputation will change it. The rejection is permanent and the message is not stored on Mimecast’s side, so it has to be sent again once the policy no longer covers you.
Common causes
- The company decided not to accept mail from your address or domain.
- A broad policy, such as one covering a whole sending platform’s domain, caught your mail along with others.
How to fix it
- 01Confirm whether the block names your Header From or your envelope sender, from the wording.
- 02If the contact is wanted, ask the recipient to have their IT team change or remove the Blocked Senders policy.
- 03If your sending platform’s envelope domain is the one blocked, use a custom return-path domain of your own.
- 04Resend after the policy is changed; the rejected message was not kept.
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.
- 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.
- 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.
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.
All 19 Mimecast codesEvery code in the wikiPaste your own bounce