Is it better to send a job rejection letter by mail or email?

It is more professional to send a job rejection letter by mail. Business protocol requires a printed copy of important business transactions. However, if all of your correspondence with the company has been by email, you may send your rejection letter by email as well. If you were sent a job offer by mail, that should be your cue to send your job rejection letter by mail as well. If you were offered a job by phone, you should still err on the side of being too polite and send your rejection letter through the mail.