Mimecast rejected your message

Message rejected prior to data acceptance

To Address (Post checks): Rejected prior to DATA acceptance

This is a stage, not a cause. Mimecast refused the message before the sending server transmitted its content — at or before the DATA step of the SMTP conversation. Because no content was ever accepted, there is nothing held anywhere to release; the message exists only as a line in the rejection log.

Mimecast does not publish a page defining this exact phrase. What its documentation does establish is that rejections at this stage come from the connection and envelope checks: IP reputation and blocklists, recipient validation, blocked-sender policies and greylisting. The reason text paired with it in the log or bounce names the check that actually fired; this phrase alone does not.

Who fixes it: It depends on the paired reason — that string, not this one, says whether the fault is your reputation, your recipient data or their policy. Decode the full bounce rather than this fragment.

Greylisting: the first-contact refusal

Attempt Greylisted

Greylisting is Mimecast's default treatment of senders it has not seen before. On the first connection it records the triplet — sending IP, envelope sender, envelope recipient — answers with a 451 temporary failure, and waits. A real mail server retries; most spam software does not.

Mimecast publishes the window: the retry must arrive between one minute and 12 hours after the first attempt. Any RFC-compliant mail server retries well inside that, so the usual experience is a delay of a few minutes to an hour on the first message to a domain, then nothing — once a triplet has been accepted it is remembered. If the sending server never retries, Mimecast logs Sender Failed to Retry and the message is simply lost.

Who fixes it: The sender's, but only in the sense that your mail server must retry — which any properly configured one does on its own. If greylisted messages never arrive, investigate why your server does not retry, not how to make Mimecast stop.

Expired in queue: rejected by housekeeping

Expired in queue - Rejected by housekeeping

Housekeeping is Mimecast's automated queue clean-up, and this message means it gave up on a message that sat in a queue past its lifetime. Mimecast does not document the exact string publicly; here is what its documentation and support answers do establish.

Two queues produce it. A message parked in the held queue for review is removed by housekeeping if nobody acts on it within 14 days — the review simply never happened. A message in the delivery queue is retried 30 times over four days; if every retry fails, housekeeping expires it. Either way the queue entry is gone and nothing further will be attempted, though a message Mimecast had accepted remains searchable in the archive.

Who fixes it: The recipient's, in the held-queue case — their administrator has to act on held mail before housekeeping removes it. In the delivery case the expiry is only the tombstone: the real fault is whatever made every retry fail, and that earlier deferral reason is the thing to decode.

Envelope blocked, envelope rejected

550 Envelope blocked - User Entry

550 Administrative prohibition - envelope blocked

An envelope rejection means Mimecast refused the message on its envelope sender address — who it claimed to be from — before looking at the content. Mimecast's documentation is direct about the cause: a Blocked Senders policy at the recipient organisation matched your address or domain.

The variant names who created the entry. User Entry means an individual at the recipient blocked your address from their own mailbox — one person's decision, not company policy. Administrative prohibition, and the plain Envelope Rejected log entry, point to a policy an administrator configured.

Who fixes it: The recipient's, entirely. Mimecast's own remediation is to delete or modify the Blocked Senders policy, and only they can do that. If one contact's mail matters, ask them by another route to unblock you; sending more mail to the same address changes nothing.

DMARC sender invalid

550 DMARC Sender Invalid - envelope rejected

This one is not Mimecast's opinion of your mail — it is Mimecast enforcing your own domain's published DMARC policy. The message failed DMARC: neither SPF nor DKIM produced a pass aligned with the domain in your From header, and the recipient's DNS authentication policy, which honours the sender's DMARC record by default, rejected it as your policy instructs.

That makes it the most fixable rejection on this page, because everything involved is under your control: the SPF record, the DKIM signing, and the DMARC policy they feed into. It is also the one where asking the recipient to whitelist you is answering the wrong question.

Check your domain: the SPF, DKIM and DMARC checker shows whether your records pass and align, and the exact record to publish if they do not.

Who fixes it: Yours, in DNS. Fix alignment so SPF or DKIM passes for your From domain. Nobody at the recipient needs to be involved, and once the records are right the same mail clears every gateway, not just Mimecast.

RWL, permitted senders and greylist bypass

Mimecast's connection logs sometimes credit a skipped greylisting check to an RWL — a realtime whitelist, the mirror image of the RBL blocklists. Mimecast does not document its RWL sources publicly, so nothing more specific can honestly be said about which lists it consults.

What is documented: senders in the recipient's Auto Allow database or a Permitted Senders policy are not greylisted, a Greylisting policy can exempt senders explicitly, and permitted senders also bypass reputation and spam scanning.

The part senders regularly get wrong: every one of these bypasses is configured by the recipient, inside their Mimecast console. There is no form a sender fills in, no whitelist you can apply to join, and no way to buy your way past greylisting. A first-time delay to a Mimecast-protected domain is normal and self-resolving.

Who fixes it: The recipient's, if anyone's — and usually nobody's. Greylisting resolves itself when your server retries; ask a contact for a Permitted Senders entry only if your mail to them is repeatedly delayed or blocked.

The 451 internal-resource deferral

451 Internal resource temporarily unavailable - https://community.mimecast.com/docs/DOC-1369#451

Despite the wording, this is rarely a Mimecast outage. Mimecast's error-code documentation gives it two meanings: the sending server is being greylisted — see above — or the sending IP's reputation is poor enough that Mimecast is deferring rather than accepting.

It is a 451, so it is temporary by definition and your server retries on its own. Greylisting clears on the retry. A reputation deferral that persists across many retries is the more serious reading, and the pattern distinguishes them: greylisting affects the first message and then stops; reputation trouble affects everything.

Who fixes it: Usually nobody's — let the queue retry. If deferrals persist for hours across many messages, treat it as a sender-reputation problem: check your IP against the public blocklists and confirm SPF, DKIM and DMARC pass.