Problem Reported:
The mail sent within the same domain got quarantined/blocked on the SpamTitan.
Observations:
Check the mail in Spamtitan > Report > History
Check the score, check due to which Rule score has been increased. You can check that in 'View Source' option.
If the rule is, tests=[ANTISPOOF_DOMAIN=25 the mail is blocked in spoofing as the Sender and Recipient domain are same.
Solution:
Add the sender domain in SpamTitan domain allow list