# SMTP 450: requested mail action not taken, mailbox unavailable

URL: https://iblockspam.com/smtp-errors/450
Last updated: 2026-10-09

Code: 450 (temporary failure, the sender retries)
Provider: Standard (RFC)

The receiving server couldn't deliver to the mailbox right now, for example because it is busy or temporarily blocked by policy. The sender should retry later.

What the bounce says:

```
450 4.2.1 The user you are trying to contact is receiving email too quickly. Please resend your message at a later time.
```
Gmail's text for a recipient that is receiving mail too fast (start of the message as published by Google).

RFC 5321 describes 450 as "Requested mail action not taken: mailbox unavailable (e.g., mailbox busy or temporarily blocked for policy reasons)". It is the temporary twin of 550. The server is saying: not now, but this might work later.

Two situations produce most 450 replies. The first is a recipient mailbox that is receiving more mail than its provider allows in a short time, which Gmail reports as 450 4.2.1. The second is a policy deferral. RFC 6647, which describes greylisting, notes that the reasonable choices for a greylisting delay are 421 if the server closes the connection, and 450 otherwise. A greylisting server deliberately defers a sender it hasn't seen before and accepts the message when a properly behaving server retries.

Because a 450 is temporary, your mail server should handle it on its own. You only need to act if the same recipient keeps getting 450 for many hours, or if the text points to a policy problem on your side.

## Common causes

- The recipient's mailbox is receiving mail faster than its provider allows.
- Greylisting: the receiver defers senders it hasn't seen before.
- The mailbox is locked or busy while the provider does maintenance.
- A temporary policy block on your IP address or domain.

## How to fix it (sender)

- Let your server retry on its normal schedule. A greylisted message is usually accepted on a later attempt.
- If you send automated mail to one recipient, slow down the rate.
- If the deferral lasts more than a day, contact the recipient another way and share the full error line.

## How to fix it (recipient or admin)

- Check whether the mailbox is receiving a mail storm, such as an automated system or a forwarding loop.
- If your server uses greylisting, add regular partners to its pass list so their mail isn't delayed.

**Q: What is the difference between SMTP 450 and 550?**
Both mean the mailbox is unavailable. A 450 is temporary: the sender keeps the message and retries. A 550 is permanent: the message bounces and resending it unchanged will fail again.

**Q: Is a 450 error a sign of greylisting?**
It can be. RFC 6647 says a greylisting server should return 421 if it closes the connection and 450 otherwise. If the first attempt gets a 450 and a retry a few minutes later succeeds, greylisting is the likely reason, and nothing needs fixing.

Sources:
- RFC 5321: Simple Mail Transfer Protocol (reply codes, section 4.2): https://www.rfc-editor.org/rfc/rfc5321#section-4.2
- RFC 6647: Email Greylisting, an applicability statement for SMTP: https://www.rfc-editor.org/rfc/rfc6647
- Gmail SMTP errors and codes (Google Workspace Admin Help): https://knowledge.workspace.google.com/admin/support/troubleshooting/gmail-smtp-errors-and-codes

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