HeLP with mail issues after selling iPhone 3GS!!

I sold my iPhone to someone after restoring the factory settings (which i thought cleared the iphone memory) however after a day the purchaser told me that they received an email of mine on the iPhone. It doesnt make sense as i have checked the mail settings and it shows their email address and not mine. Am I missing something?? PLz help as i dont want my personal messages being sent to other people! thanks in advance...