Among the common mail server errors, 421 is a common one that happens due to ‘too many concurrent SMTP connections’. This error usually occurs a high-traffic recipient mail server.

What causes ‘Too many concurrent SMTP connections’ error?

Senders sending mails to a high traffic mail server can end up getting bounce mails. The error message in the bounce mail would usually be ‘421 Too many concurrent SMTP connections; please try again later.’

The error message denotes that the connections to the recipient mail server have exceeded the limit. This high traffic can sometimes be a temporary problem due to a spike in the connections.

But there maybe some other situations where the traffic is dangerous and expert intervention is required. For instance, spammers can hack email accounts and send bulk mails.

There could be some websites that have mass mailing scripts such as newsletters. Virus or malware in the server can enable the attackers to abuse the email server for spamming.

All these issues lead to a high mail server traffic and can end up bouncing mails coming to it. The solution for the error would depend on what caused the high traffic.

‘Too many concurrent SMTP connections’ error in Exim mail server for cPanel and DirectAdmin

cPanel and DirectAdmin are the two commonly used control panels in web hosting industry. The default mail server they use is the Exim.

In Exim mail server, there are many parameters to ensure ‘Resource control’. Resource control is crucial in a shared server where multiple users share the resources.

Without proper resource limits in place, one user or application can take up the entire memory for that process and cause the mail server to crash.

Some important resource control parameters used to manage Exim in cPanel and DirectAdmin servers are:

smtp_accept_max-maximum no. of simultaneous incoming connection

smtp_accept_max_per_connection-maximum no. of message per connection

smtp_accept_max_per_host-maximum no. of  connection from one host

smtp_accept_queue-queue mail if more connections

One of the commonly used setting to limit Exim resources is the ‘smtp_accept_max‘ parameter. It specifies the maximum number of simultaneous incoming SMTP connections that Exim will accept.

If the ‘smtp_accept_max limit has been reached for Exim connections, due to the high traffic, further connections to the mail server are rejected.

For instance, in this server, the limit is set to 100.

[email protected] [~]# cat /etc/exim.conf |grep smtp_accept_max  smtp_accept_max = 100

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