451-The server has reached its limit for processing requests from your host.
Today one of our client complained about receiving below error when he was sending emails from his server1 to server2.
SMTP error from remote mail server after initial connection: host domain.com [xx.xx.xx.xx]: 451-The server has reached its limit for processing requests from your host. 451 Please try again later.: retry timeout exceeded
It’s due to maximum incoming smtp connection limit(smtp_accept_max) set on the server2.
Login to WHM >> Exim Configuration Manager >> click on “Advanced Editor” and search for value smtp_accept_max. The default cPanel value is 100, change it 150 or 200 and save the settings.
smtp_accept_max = 200
On normal servers simply edit the exim configuration file /etc/exim.conf file and set the above value.