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SMTP Error 421: What is This Error and How Can I Fix it?

You've encountered 'SMTP Error 421'

SMTP (Simple Mail Transfer Protocol), is the standard protocol for exchanging emails online. This system ensures your messages are routed to the correct mail servers and, finally, to the intended recipient. As with everything, no system is perfect, and you may encounter errors with SMTP, such as the common SMTP Error 421. We’ll explain what this error is and things you can do to fix it.

What is SMTP Error 421?

This problem occurs when your messages are being deferred by the receiving mail servers for some reason. The recipient server is restricting your messages and preventing their delivery. It usually happens if there are too many simultaneous connections to the server.

What Causes SMTP Error 421?

This error signals a problem with your mail server’s connection to that of the recipient. The primary cause is too many connections or a large volume of messages sent simultaneously to the recipient server. However, there are various other causes, including

  • Configuration mistakes in your SMTP software.
  • An incorrect SMTP port number is being used to initiate a connection.
  • Corrupted data files.
  • Network interference from antivirus software, firewalls, virtual private network (VPN) services, etc.

How To Fix SMTP 421 Errors

1. Change Port Settings

By default, the Simple Mail Transfer Protocol uses port 25 to initiate a handshake between your email client and server. If this port is misconfigured or blocked, you’ll likely encounter the 421 error and won’t be able to send messages.

You can head to your SMTP provider’s settings and change the port number from 25 to another usable port; 465, 587, or 2525. This will solve the problem in some cases.

2. Re-Add Your Account

You may encounter this problem with an email account you just added to your client. In this case, you can remove and add the account again to reset the misconfigured settings.

3. Disable Firewalls

Many people install firewall software on PCs to filter network traffic based on defined security policies. Your firewall software could be blocking specific functions of your email client and causing delivery errors. In this case, you’ll have to disable the firewall temporarily to fix the issue.

Similarly, other network-related applications, such as a VPN service or antivirus software, could be interfering with your email client. You can disable them temporarily to solve the issue.

4. Fix Corrupted PST Files

Personal Storage Table (PST) is a file format that you’ll encounter on email clients using Microsoft Exchange Servers. This format enables the client to store copies of messages retrieved from the server.

Sometimes, PST files get corrupted and cause mail delivery issues. In that case, you can use your client’s repair tool to scan for corrupted files and fix any ones you detect.

Other SMTP Errors

There are several other SMTP Errors apart from 421. They include

  • 250: Message not delivered because the recipient’s mailbox is invalid.
  • 451: You have exceeded the limits for sending emails from your server. These limits are set to prevent abusive behavior, such as spamming.
  • 452: Not enough storage on the recipient’s server. Your message hangs until enough storage becomes available.
  • 550: Receiving mailbox is not available.
  • 553: The mailbox you sent the message to does not exist.
  • 554: Receiving server refuses to accept the email. This can occur for various reasons, e.g., the mailbox has been suspended or is invalid.

We have explained what SMTP Error 421 is in detail and the steps you can take to solve it. It’s not easy to pinpoint the exact source of the issue, but following all the steps we mentioned will likely solve it.


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