Anyone can send e-mail from any address. It's that simple.
On MSN, however, e-mail addresses have to be verified. If you see (E-Mail Address Not Verified) after someone's name, it probably means they don't actually own the address they are using on msn. For example, if someone has @sythe.org with (E-Mail Address Not Verified) after it, they might not be from sythe.org at all.