Error «421 Too many concurrent SMTP connections»

Symptoms

Web applications display an SMTP server error:

Invalid greeting. response=421 Too many concurrent SMTP connections; please try again later.: 421 Too many concurrent SMTP connections; please try again later

Causes

The SMTP server connection limit has been exceeded.

Solution

The connection limit can be increased by changing the smtp_accept_max parameter in the Exim configuration file at /etc/exim4/exim4.conf.template (for RHEL-based systems the path is /etc/exim4/exim4.conf).

After making the changes, restart the Exim mail server service:

service exim restart