This error generally occurs due to smtp_accept_queue_per_connection limit, By default cpanel allow maxi. 30 mails in queue per connection, So increase smtp_accept_queue_per_connection limit in exim.conf to resolve the issue.
Open exim.conf And find the line smtp_accept_queue_per_connection.
#vi /etc/exim.conf
smtp_accept_queue_per_connection = 60
#service exim restart
That’s it.
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