Wirelessly posted (Mozilla/5.0 (iPhone; U; CPU iPhone OS 3_0 like Mac OS X; en-us) AppleWebKit/528.18 (KHTML, like Gecko) Version/3.1.1 Mobile/7A259g Safari/525.20)
Attacments don't get downloaded unless you click on them. So someone can send you a 100mb file and the mail app won't care unless you explicitly open it from the phone, at least with IMAP. The email is stored in a database but not sure where it keeps fetched attachments. Probably a system folder?