Tags: Technical


In the first of a series of blog posts where we attempt to demystify the most common SMTP error codes, today we discuss AOL's 521 error code:

521 5.2.1 : (CON:B1) http://postmaster.info.aol.com/errors/554conb1.html"

What does it 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 blacklist. Solution: Locate the account that is sending spam, and shut it down.

You could also sign up for MailChannels Cloud, which is a cloud-based SMTP relay service that detects and blocks spam.

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 smtp.gmail.com. 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.

Need help with your mail servers?

Contact us if you have any questions about preventing these errors.

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