I am using InfoPath 2007.
InfoPath 2007 is case sensitive when comparing text fields.
"A@msn.com" != "a@MSN.com"
To compare these two text values, I used the following expression:
translate("a@MSN.com", "ABCDEFGHIJKLMNOPQRSTUVWXYZ", "abcdefghijklmnopqrstuvwxyz") = ...