Mimecast 451 Recipient temporarily unavailable
451 Recipient temporarily unavailableMimecast temporary recipient deferralTemporary: the sender retriesFrom Mimecast
451 Recipient Temporarily Unavailable.
What it means
The 451 makes this a temporary condition, so your server keeps the message and retries, unlike a 550 Invalid Recipient, which is final. Mimecast’s published SMTP error reference gives two reasons for it: the sending IP address was placed on a block list for making too many invalid connections, or the recipient address validation Mimecast relies on is not responding.
The block list case is about your connections, not the recipient. Mimecast counts invalid connections from an IP, such as attempts to many addresses that do not exist or unauthorized relay attempts, and defers further mail from that IP for a while. The validation case is on the customer’s side: Mimecast sits in front of the customer’s real mail server and checks addresses against a directory, and when that check does not answer it asks you to come back later rather than refuse a possibly valid address.
If every Mimecast customer defers your mail at once, look at your own sending first. If only one domain defers while others accept, the customer’s recipient validation is the likely cause.
Likely causes
- Your IP made too many invalid connections to Mimecast, for example a list with many dead addresses at Mimecast customers, and was put on a temporary block list.
- Your server sends outbound or journal mail to Mimecast from an IP that is not yet authorized, which counts as invalid connections.
- The recipient address validation Mimecast uses for that customer, such as a directory sync or recipient lookup, is not responding.
How to fix it
- Leave the message in your queue; your server must retry, and most of these clear within hours.
- Check your own logs for a burst of rejected recipients or relay attempts toward Mimecast hosts and clean the list or fix the route before the retries pile up.
- If it persists past a day for one domain only, contact the recipient another way and send them the deferral text.
- The recipient’s admin should check Mimecast’s delivery queue for their domain and the health of their mail server or connector.
- Once fixed, your queued message delivers on the next retry. No resend is needed unless it already expired.
Checks that apply
- MX Lookup Confirms the domain routes through Mimecast, so you know the deferral is Mimecast speaking for the real server.
- MailFlow Monitor Sends test messages and tracks when they arrive, showing whether the deferral is still happening.
FAQ
- Is the address invalid?
- Not necessarily. A 451 means Mimecast could not decide right now; an invalid address gets a 550 Invalid Recipient instead.
- Is this my fault as the sender?
- Sometimes. Mimecast also uses it when your IP made too many invalid connections. If the recipient’s address validation is the cause, the fault is on their side and retrying is all you need to do.
Sources:Policies: Mimecast SMTP Error Codes (Mimecast Support)Checked 2026-10-18