# Error: 450 Requested mail action not taken: mailbox unavailable

Source: https://catalystgroup.gg/docs/email-delivery/error-450-mailbox-unavailable/
Updated: 2026-10-19 · By Duane Garner

The server could not handle the mailbox right now and asked you to try later. Usually temporary; persistent cases point to DNS or policy.

> **Shown by SMTP-based sending (Other SMTP, Zoho Mail, the backup SMTP transport):** The test result or the error log shows a reply starting with `450`, for example `450 Requested mail action not taken: mailbox unavailable` or `450 4.7.1 Client host rejected`.

 

This article is for site owners sending through an SMTP server who get a 450 reply.

 

## What this error means

 

Reply code 450 means the server could not act on a mailbox at this moment. It might be the recipient's mailbox, or a check on your sender address. The 4 at the start makes it temporary: the server expects you to try again later. The text after the code, and the three-part code such as 4.7.1 or 4.1.8, tells you which check failed.

 

## Common causes

 
- The recipient's mailbox is busy, locked or being moved.
- Greylisting: the server refuses the first attempt from an unfamiliar sender and accepts a retry a few minutes later.
- `4.7.1 Client host rejected`: your web server's IP has no reverse DNS name, or a name that does not match, so the server is wary.
- `4.1.8 Sender address rejected`: the domain in your From address has no valid DNS records (no MX or A record).
- A temporary policy block because of your server's reputation.
- The recipient's server is having a problem of its own.

 

## How to fix

 
1. Retry after a few minutes
  Go to **CRM-NG SMTP → Tools → Email Test**, fill in **Send To** and press **Send Email**. Greylisting and busy mailboxes clear on a retry.
2. Read the full reply
  Open the error in **CRM-NG SMTP → Tools → Debug Events** and note the code after 450.
3. Fix the sender domain
  For a sender address problem, check **From Email** on **CRM-NG SMTP → Settings → General**. Make sure it uses a real domain you own, with MX and SPF records published.
4. Fix reverse DNS
  For a client host rejection, ask your web host to set a reverse DNS (PTR) name for your server's IP that matches a name pointing back to it.
5. Try a different recipient
  Test with an address at another provider. If that works, the problem is on the recipient's side.

 

## Verify it worked

 

The test page shows "Test email sent successfully! Check your inbox to confirm delivery." Messages to the original recipient go through on later attempts.

 

## Notes

 
- Sending from a shared web server directly is a common reason for reverse DNS and reputation problems. A dedicated mail service, through an API mailer or its SMTP relay, avoids them.
- If a 450 turns into a 550 for the same address, the problem has become permanent. See the 550 articles.

 

## Related

 
- [Other SMTP mailer guide](https://catalystgroup.gg/docs/email-delivery/mailer-other-smtp/)
- [Zoho Mail mailer guide](https://catalystgroup.gg/docs/email-delivery/mailer-zoho/)
- [DNS records for email](https://catalystgroup.gg/docs/email-delivery/dns-records/)
- [Sender name and address](https://catalystgroup.gg/docs/email-delivery/sender-name-and-address/)
- [Error: 550 Requested action not taken: address does not exist or was rejected](https://catalystgroup.gg/docs/email-delivery/error-550-recipient-rejected/)
- [Troubleshooting email delivery](https://catalystgroup.gg/docs/email-delivery/troubleshooting/)