Talk to our team

Documentation

✉️ Email Delivery

Documentation, reference material and tutorials for CRM-NG SMTP Email: setup guides for every mail service, troubleshooting, and one article per error message.

Error: 450 Requested mail action not taken: mailbox unavailable

Updated

By Duane GarnerChief Executive Officer (CEO)
Summarize: Claude ChatGPT View Markdown
On this page

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.
Go to Top