I am also looking for a way purge the email addy autofill of old contacts that no longer exist. Many a family and friends have since changed their E-mail addy's, I have deleted their old addy's from contacts, yet the iPhone autofill still manages to hang on to those in some stored cache that is NOT accessible to purge or delete! This is something I've struggled with regarding the iPhone since my first iPhone months after it originally released years ago, have since owned a few other iPhones as they progressed, currently an iPhone 4, on os 4.01, same issue persists. Have restored my phones, started from new during some restores, etc, those old defunct E-mail addy's still autofill!
I sync with my iMac yet do not even have my mail client set up on the iMac, never have since we bought the iMAc, web based E-mail only, so this issue has nothing to do with mail client on the computer it syncs to.
It would be nice if someone from Apple would reply with acknowledgment that they are indeed aware of this and hopefully it is on their to-do list.
Regards,
Paul