You might come under a situation where your client expects you to secure their mail server and add a limit to the recipients they can send an email to, in order to stop spam emails.
Solution:
nano /etc/exim.conf
recipients_max = 10
recipients_max_reject = true
service exim restart
nano /etc/exim.conf
recipients_max = 10
recipients_max_reject = true
service exim restart
Hope this article helps!