When setting up CPANEL customers, even if the initial test message works. As soon as you change the MX record you may see the following bounce message when sending to the destination server like the following:
SMTP Response:host 22.214.171.124[126.96.36.199] said: 550-Please turn on SMTP Authentication in your mail client. 550-cloudd.spamtitan.com [188.8.131.52]:51608 is not permitted to relay 550 through this server without authentication. (in reply to RCPT TO command)
We have found this article which has shown a solution for a similar issue with CPanel and Spamtitan.
From this article check the following details:
If you want only to scan *incoming* emails on SPAM/Junk then all you
need it to point DNS MX records for your domains to spamtitan appliance.
Just make sure you select "Local Mail Exchanger" in "Email Routing"
section during DNS zone modification. It will put domains to
/etc/localdomains, thus Exim will still consider domains as local and
allow your scripts to send emails without sender verification failed