Hi soggy,
Could just be spam, but it possibly could be what is known as a "Joe Job", which is much more serious.
"Essentially, a Joe Job is a very crude form of identity theft. Your email address is used as the "sender's address" in most cases, and your Website URL is advertised, but an especially diligent and vicious attacker may even use your name in the signature of the message. The email will not only be sent to thousands, hundreds of thousands, or millions of addresses, but it will be sent multiple times -- possibly dozens or hundreds -- to each recipient before the attack ends.
You will first become aware that your site is the victim of a Joe Job by receiving a few bounces when you check your email."....
More info
HERE