The mailbox exists, so why does Mimecast refuse it?

Mimecast checks its own synchronized directory. If the mailbox was created after the last sync, it is unknown to Mimecast until the next one.

Is this the sender’s problem or the recipient’s?

If the address is mistyped, it is the sender’s. If the address is correct and new, it is the recipient admin’s, who needs to update the directory.

Mimecast 550 Invalid Recipient: address not recognised

550 Invalid RecipientMimecast recipient validation failurePermanent: retrying will not helpFrom Mimecast

Example

550 Invalid Recipient

What it means

Mimecast keeps its own list of valid addresses for each customer, built from directory synchronization with Active Directory or Microsoft Entra ID, or from addresses it has learned. When the RCPT TO address is not on that list, it refuses it with “550 Invalid Recipient”. Newer responses may add a link to Mimecast’s documentation after the text.

The message never reaches the customer’s real mail server. That matters, because the address might exist in Exchange or Microsoft 365 and still be refused here if Mimecast has not caught up. So this bounce has two quite different meanings: the address is wrong, or the address is new and Mimecast does not know it yet.

This is the Mimecast counterpart of a generic 5.1.1. It differs from “Envelope blocked”, where the recipient is valid but you are blocked.

Likely causes

A typo, an old address, or a person who has left the organization.
A brand new mailbox, shared mailbox, or alias that directory sync has not imported yet.
An address that exists only on a system Mimecast is not synchronized with, such as a separate on-premises server.
A distribution group or contact that the sync scope filters out.

How to fix it

As the sender, check the address against the person’s signature or a recent message, then ask them directly if it still looks right.
As the recipient’s admin, search for the address in Mimecast’s directory view; if missing, run a directory sync or add it by hand.
Confirm that the sync scope includes the organizational units, groups, and shared mailboxes that receive external mail.
After the fix, ask the sender to resend; the original attempt was rejected permanently and will not retry by itself.

Checks that apply

Your domain or sending IP (fills in the checks below)

MX Lookup Shows the MX points at Mimecast, which tells you the refusal came from Mimecast’s recipient check, not the mailbox server.
SMTP Diagnostics Connects to the Mimecast host to confirm it is reachable and presenting the expected banner and certificate.

FAQ

The mailbox exists, so why does Mimecast refuse it?: Mimecast checks its own synchronized directory. If the mailbox was created after the last sync, it is unknown to Mimecast until the next one.
Is this the sender’s problem or the recipient’s?: If the address is mistyped, it is the sender’s. If the address is correct and new, it is the recipient admin’s, who needs to update the directory.

All 191 bounce codes