We can restrict sending and receiving mails which are exceeding a certain size limit with cPanel by adding the line "message_size_limit" on exim configuration file(/etc/exim.conf).
Example:
We can restrict sending and receiving emails to a size limit of 20MB by adding the following line in the first part of your exim.conf file.
#vi /etc/exim.conf
message_size_limit = 20M
This setting should restrict both incoming and outgoing emails with in 20MB.
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