if your mail is typed in UTF8 while the other side is reading in GB2312, gibberish will result. check with the encoding format your friends use to open the mail.

i'm not very sure abt the difference between encoding and charset. but i think, ideally, if everything is encoded with UTF8, no problem will occur.

if you have problem reading your own round-trip mails, it might mean the mail is corrupted in transit or the encoding has been altered.