The client is an iPhone.  I'll have this user dig into his configuration settings to see if he can identify where it is coming from.

I have a user who connects via IMAP and when he forwards a message the Resent-From header appears as localuser@domain.com rather than domainuser@domain.com.  Where is Resent-From header being populated from?

There is no such header added when I forward a message, so it's presumably client-specific.