Looks like a compatibility problem with JavaMail and your client. What client?
Well it's the same in outlook, gmail and squirrelmail. It is also the same whether i send it from my production server (Jboss 4.0.5. Linux, ) or my desktop (JBoss 4.0.5, Windows) which are using different smtp servers.
If i don't use the <m:headers> I don't get this ------=_Part..etc texts, but instead my emails looks like they have an attachement, which they do not have.
Which version of Seam?