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Best Practices

Why Did I Get “521 5.2.1 : (CON:B1)” Error?

By Ken Simpson | 2 minute read

Have you been seeing the following AOL’s 521 error code when trying to send your mail?

521 5.2.1 : (CON:B1)"

What does this error message mean?

If you received this error message in your inbox, it means that you tried to send someone at AOL an email message, and AOL’s mail server rejected the connection because it’s seen just a bit too much spam lately from your mail server.

What can I do?

If you are seeing this error message, you need to talk to your email administrator. In all likelihood, your mail server is sending spam on behalf of a compromised user account (i.e. an account which has been taken over by spammers). The spam traffic is making its way to AOL, and AOL has added your mail server’s IP address to an internally maintained AOL IP blocklist.

Solution: Locate the account that is sending spam, and shut it down

What else can I try?

If you can send through a different mail server, that might help to get your email delivered more reliably. For example, if you have a Gmail account, use the Gmail SMTP server to send your mail. Its host name is You’ll need to enable TLS security, and provide your username and password in order to use Gmail’s SMTP server. More information can be found on the Google Support Site.

MailChannels Outbound

MailChannels Outbound is an email relay service that identifies spammers within your network to improve your email reputation and ensure reliable email delivery for your users. With MailChannels Outbound, you can eliminate email delivery problems caused by IP address blocklisting. You can also automatically shut down compromised end user accounts and scripts to improve your security.

Check out our whitepaper, “What Is An SMTP Relay Service?”.

Editor’s note: This post was updated: July 20, 2020

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