You need to call Sprint and have them reset it. If you cannot remember it, then that is the only option.

And BTW, some of us do not recommend using the carrier supplied email account for 3 main reasons: (1) no SPAM filter whatsoever (2) when you change carriers or cancel your BIS account, that address is gone forever and (3) the only way to read email coming to that account is on the BB, so in a data outage, you cant receive/send/read email.