Spam emails from guests

[Nemo]

I've reposted this from the members only section as I didn't intend to post there.

Ok, I've had this problem before and it has reared it's ugly head again.

Here was my first thread:

viewtopic.php?t=16569

Last week I had two guests send me emails containing spam. I contacted them and one was aware that his email had been hacked and had changed his password. Then my own email sent me a spam email, copied to the same email. I immediately changed my password. I have received 3 more spam emails from three different guests yesterday and another one today.

I now have a new computer plus my existing laptop and both computers are connected to a home network which has a back up. The spam is coming from guests to my work email and I can pick them up on my iphone before I even open them on my computer. Is the problem at the server end? Do I need to contact my hosting company for help?

Well in over my head as usual!