If it's sent to you directly and without assumption of confidentiality, yes; as with any form of communication.
Even with the assumption of confidentiality, it wouldn't be against the law to post the material.
If you signed a contract stating you wouldn't post the information and you went ahead and posted it anyway, they could sue you for breach of contract and effectively garnish a sum equal to the damage caused to them by its posting.
But an e-mail sent to you without the proper procedure of copyright or prior contract obligation is as much yours as it is the sender's.
