421: sending IP blocked by a Blocked Senders policy

What it means

Mimecast answers mail for the company you are writing to, and that company’s administrators decide what its policies block. Mimecast documents this reply for a Blocked Senders policy that matches the IP address of the server delivering your message. The code starts with 4, so your server treats it as temporary and keeps retrying, but each attempt meets the same policy. Nothing changes until someone on the recipient side edits or deletes the entry. The block sits in one customer’s configuration, so mail from the same IP to other Mimecast customers is not affected by it.

Common causes

• An administrator at the recipient company added your IP address, or a range containing it, to a Blocked Senders policy.
• You send from shared infrastructure, such as a bulk sending platform, whose IP was blocked because of someone else’s mail.
• An old block from an earlier incident was never removed.

How to fix it

1. 01Note the IP address your mail left from; the bounce or your sending platform’s logs will show it.
2. 02Contact your recipient through another channel and ask their IT team to remove that IP from their Mimecast Blocked Senders policy.
3. 03If the IP is shared and you cannot get it unblocked, send from a different, well-kept IP or provider.
4. 04Stop retrying in bulk until the entry is gone, since every attempt is refused the same way.

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