Two examples where a user might want to forward a mail:
- a user wants to forward an example of abuse/spam that they have received to an admin profile
- two friends want to share with each other an interesting/amusing mail which they have received from someone (I have this situation, for example, where I am also a member of my site, as well as a friend of mine as a different profile, and when I get an amusing/nice/interesting mail from someone I have to print screen then copy/paste to a "real" email program like Outlook, then send it there! This is ridiculous)
Treat people as adults! Give them the opportunity to forward mails if they want.