Cannot access webmail when Qmail on is used on Plesk: possibly qmail-smtpd exited by timeout

Symptoms

Qmail is used as mail server on Plesk.

Cannot access webmail, the following error message in /var/log/maillog:

qmail-queue-handlers[19027]: possibly qmail-smtpd exited by timeout, reset connection. #011#011#011#011 See "http://pobox.com/~djb/docs/smtplf.html." for get more information.

Cause

Insufficient number of Maximum number of connections and Maximum number of connections for a user per IP address.

Resolution

1. Log into Plesk.
2. Navigate to Tools & Settings > Mail Server Settings.
3. Increase "Maximum number of connections" and "Maximum number of connections for a user per IP address".