It's late and I can't see your problem, I guess, but a couple notes.

You're liable to get the On Useless Cats award of the week from Merlyn.   Why would you say "cat file | grep 'thing'" when you could just say "grep 'thing' file" ?

And when you do search for a user name in users.txt, wouldn't you like to make sure you get the _right_ user?   Suppose there's a user 'roo' - you wouldn't want to delete 'root' by mistake would you?   You should be more careful in your grep to limit how you match things.   Even using '-w' isn't good enough.   What if there were an email address "roo@where.ru" for some other user?

Could it be that some of your email addresses are more complex than just "user@domain.tld"?   Maybe you should accept a larger set of email address patterns?

Any chance you could have casing problems, such that a user name in one of the three locations, %postiniList or %ldapList or users.txt, could be upper- vs. lowercase elsewhere?

You might have to show us more code and more of the actual data.   And what does "messing it all up!" mean?