Data mask of email body that creates the case(incoming email registration process)

The case gets created when a mail is send to the setup mail of the system, now that mail is added to the conversation tab (freedom ui) as the follow up for that particular case. Now i want to edit/masked the body content of that mail before saving or displaying in any section or tab, I can mask data using regex in script task, but can't find where to perform this operation, I even checked out the incoming email registration process but couldn't undersatnd the code even i try to take the body and then applied regex but some body not defined/missing error pops.
I also tried activity added trigger and then mask the content it is doing the thing, but not getting reflected on conversation or other places, even I try to put a timer in case creation process, if it could help but nothing work out
How to do that now?