One thing worth mentioning is that sometimes an email which purports to come from a particular account hasn't. If a friend of the account holder gets their email account and address book hacked, the emails sent out by the hacker may be sent with the identity of anyone in the address book.
They usually spam others in the same address book on the basis that two people whose email addresses are in the same address book are more likely to know each other and therefore view emails with less suspicion.
I used to get emails apparently from my own email account but that was impossible. Looking at the full email headers showed that the emails didn't come from any of my mail servers but it was just my address which was being used by a third party sent via a hacked PC.
I've now put a stop to these sort of emails by specifying which email servers may send email from my domain (SPF record for the curious). Anything sent from an unauthorized server is rejected. Some email providers also use SPF to filter out incoming messages dropping those which come from places they shouldn't.