Yes, it would be possible to fake that e-mail (it's essentially what spammers do), but to prove it would be a little harder. You might be able to get some information if you had the full headers from the mail, but even that probably won't tell you that much. If you had the mail logs from your company's mail server, you could see if the e-mail was delivered to everyone in your group, but mail logs are sometimes aren't kept for that long (2 weeks) due to the amount of mail that can go through a mail server.

Adrian