Happens all the time I'm afraid.
You can at least filter them out at your ISP's server by using Mailwasher (they are, effectively, spam) click here
There are different schools of thought in this Forum as to whether you should use Mailwasher to simply 'blacklist' or bounce this rubbish.
I'm of the bounce school of thought.
Now watch out for the 'never bounce' brigade jumping down my throat! ;-))