Particular email id is blocked on spam titan at user level.
How to remove sender id from block list at user level.
Login to SpamTitan using the Postmaster account.
Navigate to Anti-Spam Engine.
Search for the affected user (refer to the image below for guidance).
4. Once you locate the user, click on the ? icon to impersonate the user
5. While impersonating the user:
Go to Filter Rules
Click on User Block List
Locate the blocked sender ID
Click the ❌ cross icon next to the sender ID to remove it from the block list (see screenshot below).