Mail

Mail is a method by which players can instantly send messages and items. It costs credits 5 to send mail with no attachments and credits 10 to send up to eight attachments. NPCs will sometimes send mail after completing missions that involved them. Mail will remain in the mail system for a maximum of 30 days, after which it, and any items attached to it, will be deleted.

Non-subscriber restrictions
Free-to-Play players may receive mail, but they cannot send mail. Once FTP players have made a purchase at the SWTOR web store through http://buy.swtor.com, they will become a Preferred Status player and they may compose mail starting at Level 10.

Preferred Status players are only allowed to send items, not credits.