Mimecast Blocking Your Emails? Every Common 550 and 451 Rejection Explained

A Mimecast rejection tells you exactly which rule stopped your email, but only if you read past the 550. Administrative prohibition envelope blocked is a blocked-sender or SPF rule. Anti-Spoofing policy is a spoofing rule. Local CT IP Reputation is your IP's reputation. 451 Internal resources are temporarily unavailable is usually greylisting. Only 550 Invalid Recipient means the address does not exist. For most of these, Mimecast itself will not talk to you. The recipient's admin has to act.

Why Mimecast rejections are different

Mimecast is a security gateway that sits in front of a company's mailboxes. You can spot it in the domain's MX records, which point to hosts under mimecast.com. The bulk email provider lookup labels gateway domains across a list.

Unlike Gmail or Microsoft, Mimecast applies each customer's own policies at the gateway. Two companies on Mimecast can treat the same email differently. Mimecast's SMTP error code reference (read 2 November 2026) says that if you receive one of its listed errors when sending to a Mimecast customer, you should "contact the recipient's Mimecast Administrator", because "Mimecast can only deal with designated customer contacts."

For a cold sender that is a hard limit. You usually cannot ask the recipient's IT team to unblock you. What you can do is read the code correctly, fix anything on your side, and stop sending where you are blocked.

The 550 rejections, decoded

From Mimecast's reference, grouped by who can fix them:

You can fix these

Rejection	What it means	What to do
550 SPF Sender Invalid - envelope rejected	The sending IP is not listed in the sending domain's SPF record	Add your sending service to SPF. Check with the SPF checker
550 DKIM Sender Invalid - envelope rejected	The DKIM signature does not match the published key	Fix the DKIM record or selector. Check with the DKIM checker
550 DMARC Sender Invalid - envelope rejected	The message failed the sending domain's DMARC checks	Align SPF or DKIM with your From domain. See DMARC alignment
550 < details of RBL >	Your sending IP is on the blocklist named in the text	Fix the cause and request delisting from that list. See how to get off a blacklist
550 Local CT IP Reputation - (reject)	Mimecast's ongoing reputation checks rejected your IP, sometimes after earlier 4xx deferrals	Reduce volume, clean the list, fix DNS and authentication, then try again later

Only the recipient's admin can fix these

Rejection	Mimecast's explanation
550 Administrative prohibition envelope blocked	A Blocked Senders policy or an SPF hard rejection was triggered
550 Envelope blocked – User Entry / User Domain Entry	A personal block on your address or domain
550 Rejected by header-based manually Blocked Senders	A personal block policy
550 Rejected by header-based Blocked Senders - Block policy for Header From	An admin block on the From or envelope-from address
550 Anti-Spoofing policy - Inbound not allowed / Rejected by header-based Anti-Spoofing policy	The message triggered an anti-spoofing policy
550 Message bounced due to Content Examination Policy	A content rule matched something in your message
554 Host network, not allowed	A geographic restriction policy

Note that Administrative prohibition envelope blocked can also be an SPF hard fail, which is yours to fix. Check SPF first before deciding you have been blocked.

"Envelope blocked – User Entry" is the closest thing to a hard no. Someone at that company, often the recipient, blocked your address or domain personally. Do not email that person again from another domain.

These are about the address

Rejection	Meaning	What to do
550 Invalid Recipient	Mimecast's recipient checks did not find a valid internal user	Suppress the address
501 Invalid address	Not a valid SMTP address	Fix the formatting or suppress

These are the only Mimecast rejections that belong on your invalid list.

The 451 deferrals

Mimecast also uses temporary failures, which your server should retry:

Deferral	Mimecast's explanation	What to do
451 Internal resources are temporarily unavailable	Greylisting, which requires a retry between one minute and 12 hours later; or the sender's IP has a poor reputation	Let your server retry automatically
451 IP Temporarily Blacklisted	Your mail server reached a limit	Wait, then send less per connection and per hour
451 Recipient Temporarily Unavailable	Your IP was put on a block list for too many invalid connections	Stop sending to invalid addresses; verify the list
451 Unable to process connection at this time	Mimecast is under load; it retries	Nothing
421 Sender address blocked	A Blocked Senders policy has blocked your IP	Recipient admin only

The third row is worth stressing. Mimecast tracks invalid-recipient attempts per IP. Sending a batch of dead addresses to a Mimecast customer can get your IP temporarily blocked for all their users, including the real ones.

Why cold email hits Mimecast

• Greylisting. Mimecast customers often greylist unknown senders. A cold sender is, by definition, unknown. Short retry windows or tools that give up after one deferral turn greylisting into lost mail.
• Unverified lists. Invalid recipients count against your IP at Mimecast. Verify before you send.
• SPF gaps. Cold-email setups often send through a sequencer, a warmup tool and a CRM, and one of them is missing from SPF. Mimecast's SPF hard rejection catches that.
• Content rules. Some companies block messages with tracking pixels, many links, or attachments from unknown senders.

Is it a bounce or a block?

Sort your Mimecast rejections before you act on them. Paste the bounce text into Bounce Doctor, which separates bad addresses from sender blocks, security gateways and temporary failures, or into the bounce message analyzer for one message at a time.

Then:

1. Invalid Recipient: suppress.
2. SPF, DKIM, DMARC, RBL, IP reputation: fix your side, then try again after a few days at lower volume.
3. Blocked senders, personal blocks, anti-spoofing, content policy: stop sending to that domain. If the account matters, reach the person another way.
4. 451 deferrals: let the retries run, and check your tool's retry settings are not too short.

Verifying Mimecast-protected addresses

Mimecast customers can configure how the gateway handles unknown recipients. Some reject unknown addresses at RCPT TO, so a verifier gets a clear answer. Many accept everything at the gateway and decide later, which makes the domain look catch-all to any verifier. Greylisting can also make the first check inconclusive.

Our guide to verifying emails behind security gateways covers this. In short: treat Mimecast catch-all rows as risky, send them in a separate, slower campaign, and retry inconclusive rows later rather than sending blind.

Where SimpleVerifier fits

SimpleVerifier finds the invalid addresses that would earn you 550 Invalid Recipient and count against your IP at Mimecast. It also shows which domains are behind a gateway so you can segment them. It cannot lift a customer's blocked-sender policy, and no verifier can. Catch-all and inconclusive rows come back risky, never valid.

The first 100 checks are free with a Google sign-in on the verify page. Monthly plans are on the pricing page.