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Error 421: too many concurrent smtp connections

Error 421: too many concurrent smtp connections
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On high traffic email servers you may see this kind of errors, it’s very normal in some situations when your users use lot of email. It basically means that there are a lot of open SMTP connections (incoming and/or outgoing). The exact error message may be something like:

421 Too many concurrent SMTP connections from this IP address; please try again later.

How to fix Error 421 too many concurrent smtp connections?

First we have to check out what’s the smtp_accept_max value for our email server. In the case of cPanel servers, you just have to run a grep command against Exim configuration file, example:

grep smtp_accept_max /etc/exim.conf

Then edit the variable and assign more simultaneous connections. Restart Exim to apply changes:

service exim restart

Why does the Error 421 too many concurrent smtp connections happen?

As we said, there are a number of valid reasons, but also it may indicate some kind of serious problem with your server.

Valid reasons:

  • Normal high traffic: in some times of the year like holidays, special dates like july 4th, christmas or new year.
  • Newsletters: some of your users may be using massive newsletter and this can cause Exim to serve more connections than usual.

Ilegitimate reasons:

  • Coding errors: we’ve seen email loops in the code of webpages more than once, also con email client such as Outlook. This kind of behaviour can cause emails to be sent again and again, opening too many connections.
  • Virus/Malware: malware and virus are uploaded to thousand of websites each day, causing damage in networks, companies and more. And one of the most common activities that crackers love is to use your email server as mail bomb server, to send tons of emails each day.
  • Email hacking: if you are using weak passwords (like john123) on your email accounts they can be hacked easily using brute force attacks, and then used to send outgoing spam.