Bobcares.com provides Server Support to Web hosts, VPS hosts and other hosting providers.

As part of our services, we resolve tech support issues posted by hosting users. In these support issues, a common category email error we see is:

Server Response: 450 4.x.x <name@domain.com>: Reason for the error

This is usually reported by VPS users who see this error in their mail logs, or by website owners who see it in their mail clients or website diagnostics.

What is Email error 450?

Email error 450 denotes a temporary failure in mail delivery.

We’ve seen it happen during mail submission (connection from mail client to SMTP server), DNS resolution, connection to remote server and during anti-spam checks.

Usually these errors get resolved on their own, and users won’t even notice them.

But there are a few cases where the end users might see it:

• Errors shown in mail client – Typos, DNS failures or anti-spam measures in the sender’s MX can cause the mail to be rejected, and this error will be shown in the mail client.
• Web application diagnostics – When web apps like WordPress is unable to send mail, and web owners look at the connection log, they see this status code.
• While troubleshooting mail delivery – VPS owners might see this when they search mail logs to troubleshoot other issues such as IP blacklisting.

What causes Mail error 450?

Mail delivery is a complex process that involves establishing connection to the sending mail server, queuing the mail, connecting to the receiving mail server, passing anti-spam tests, and storing the mail in recipient mailbox.

If this process breaks at any of these points, error 450 is returned.

Here are a few common reasons we’ve seen for this error:

1. Security measures like Greylisting

Spammers blast off thousands of spam mails every minute. They do not care if the mails bounce or not. Whereas legitimate mail servers retry delivery even after a temporary failure.

This key difference is in behavior is used by an anti-spam method called “Greylisting”.

Receiving mail servers send a temporary failure message that causes legitimate mail servers to try again. But spammers think of it as a bounce, and abandon the mail.

We’ve seen temporary bounce displayed through status codes “450 4.7.1” and “450 4.2.0“. Some of them are:

• 450 4.7.1 <user@recipient.com>: Recipient address rejected: Greylisted for 5 minutes
• 450 4.7.1 <user@recipient.com>: Recipient address rejected: Policy Rejection- Please try later.
• 450 4.7.1 <user@recipient.com>: Recipient address rejected: Policy Rejection- Greylisting challenge offered. Legitimate and well-configured mailservers will automatically retry shortly, and mail will go through.
• 450 4.7.1 <user@recipient.com>: Recipient address rejected: Service unavailable, greylisted
• 450 4.2.0 <user@recipient.com>: Recipient address rejected: Greylisting in progress, retry after 60 seconds.
• 450 4.2.0 <sender.com[xxx.xxx.xxx.xxx]>: Client host rejected: Greylisted, see http://remotemx.com/help/greylisting/

This status code sure looks scary, and that is why many VPS owners and web developers report it as a mail error to our tech support desk.

We resolve those support issues by tracing the full path of the mail, and finding out where it is meeting with a permanent failure (often with a 5XX status code – eg. 554 5.7.1).

2. Receiver’s mailbox or domain errors

When the receiving mail server is unable to verify if either the sender or receiver is legitimate.

For eg. many mail servers perform a reverse lookup of the sending email ID’s domain name. If that domain name or the sending IP does not look like a legitimate server, the mail will be rejected.

Some common variations of this error we’ve seen are:

• 450 4.1.1 <user@recipient.com>: Recipient address rejected: unverified address
• 450 4.1.2 <user@recipient.com>: Recipient address rejected: Domain not found
• 450 4.1.8 <user@recipient.com>: Sender address rejected: Domain not found (in reply to RCPT TO command))

We’ve generally seen two reasons for this error.

• Misconfigured receiving mail server – When the receiving mail server is unable to check the user list or the domains hosted in the server, this error will be returned. It can be due to file system permissions or incorrect mail config settings.
• Non RFC-compliant sending MX – Sending mail servers must have a proper reverse DNS and must answer a verification call. If such anti-spam checks fails, receiving mail server might reject a connection.

There’s no single fix to this issue. To resolve it, we systematically go through server settings, and fix misconfigured or missing entries.

3. DNS resolution issues or Spelling errors

To send a mail, the sending domain will need to find out the IP of the receiving mail server.

If it fails for some reason (eg. unreachable DNS server), the mail delivery fails. Two variations of this issue we’ve seen are:

• 450 4.1.7 <user@recipient.com>: Sender address rejected: unverified address: Host or domain name not found. Name service error
• 450 4.4.317 554 invalid DNS PTR resource record

When users report this issue, we check if the DNS servers are working properly or if it’s unreachable due to network or load issues.

Very often these are caused due to temporary network issues, and will get automatically resolved after a while.

4. Recipient’s mail server errors

When the recipient’s mail server is under high load or is facing network issues, the sending MX might take a long time to establish connection.

It can lead to connection timeouts, and the sender will be forced to try again later. An example of one such error is:

• 450 4.7.0 too long without DATA command (closing session): retry timeout exceeded

VPS owners sometimes report this error when mails to their domains bounce with this error.

To resolve this, we look at server load history, and fix services that cause load spikes.

5. SMTP authentication not enabled

Mail servers that are armed with anti-spam measures like Greylisting considers all incoming connections are potential spammers.

Even legitimate mail account holders will be subject to anti-spam checks – unless they authenticate their mail with SMTP Auth.

In fact, almost all mail service providers require their users to use SMTP auth to send mails.

However, while handling tech support of web hosts, we’ve seen users who misconfigure SMTP auth, resulting in their mails getting rejected by mail error 450.

The mail users will see the error displayed in their mail client with error codes such as “0x800CCC79”.

We resolve these errors by providing step-by-step instructions to mail users to setup SMTP auth.

Conclusion

Mail error 450 denotes a temporary failure in mail delivery. But it can point to deeper problems within the sending and receiving mail servers. Here we’ve listed down the top 5 causes we’ve seen for email error 450 and how to fix them.