Hello,

This issue is happening on your custom SmartHost. The following custom SmartHost configuration is configured in WHM -> Exim Configuration Manager -> Advanced Editor under 'POSTMAILCOUNT" :

--
smarthost_dkim:
driver = manualroute
domains = !+local_domains
senders = !*@bison.steeldns.com
#senders = ^.*@wwubs.coma
#senders = ^.*@domain1.com : ^.*@domain2.com
require_files = "+/var/cpanel/domain_keys/private/${sender_address_domain}"
# Exclude null sender messages from relaying via the smarthost
condition = ${if or {{!eq{$sender_address}{}} {!eq{$sender_host_address}{}}}}
transport = remote_smtp_smart_dkim
route_list = $domain 5726.smtp.antispamcloud.com::587

smarthost_regular:
driver = manualroute
domains = !+local_domains
senders = !*@bison.steeldns.com
#senders = ^.*@wwubs.coma
#senders = ^.*@domain1.com : ^.*@domain2.com
# Exclude null sender messages from relaying via the smarthost
condition = ${if or {{!eq{$sender_address}{}} {!eq{$sender_host_address}{}}}}
transport = remote_smtp_smart_regular

route_list = $domain 5726.smtp.antispamcloud.com::587
--

With this custom SmartHost removed, mail sends from your server without a problem. This means that the issue is with either:

a) The above SmartHost configuration
b) Something on the SmartHost server itself

Either way, this issue is not related to cPanel. Email sends fine without the above Exim customization. I suggest getting in touch with the administrator of your custom SmartHost for investigation on what is wrong with either Exim's SmartHost configuration or on the SmartHost server itself.