If the offer is on Company’s letterhead, duly signed by authority, and scanned to you, as an attachment, though it is from personal id, it is valid, and it may be due to the reasons stated by Gia above.
Ofcourse, you must ask for a hard copy, even this is from company’s ID, since this is a very important document. Email-id in the name of Company also can be easily created by anyone.
If the offer is as a email communication directly from a personal id, it is not at all valid.