Well, I think it is a bug!
It's certainly not solved by deleting the old addresses from your iMac and then synching the phone using iCloud or via iTunes.
It seems to be a problem of the way that the Erase function still leaves the data on the memory card. Unlike the iMac, you don't have a 'secure erase' on the iPhone. If you use the 'search your iPhone' facility and type in the old address you want to eradicate, it will 'find' e-mails you sent using this old address. In fact it will 'find' any e-mails you deleted and removed from the trash. Of course it hasn't really found them, only the ghosts of the headers that are cluttering up the memory somewhere on the phone.
If these deleted e-mails and old addresses are still lurking on the memory card then what is required is an app like the Disk Utility that will securely erase all the free space by writing zeroes into all the unused memory spaces.
BTW I'm going to go and search for one - some hope!