I am using InfoPath 2007.
InfoPath 2007 is case sensitive when comparing text fields.
"[email protected]" != "[email protected]"
To compare these two text values, I used the following expression:
translate("[email protected]", "ABCDEFGHIJKLMNOPQRSTUVWXYZ", "abcdefghijklmnopqrstuvwxyz") = translate("[email protected]", "ABCDEFGHIJKLMNOPQRSTUVWXYZ", "abcdefghijklmnopqrstuvwxyz")
To test this, I used an expression box field on my test form. You should replace the first argument in each translate function with your fields that you want to compare.
Good luck.