Blocking Spam Email

If you want to find the sender you will have to look in the host headers.    But since a lot of spammers dynamically generate this it might be hard to stop.    Look in the headers:   Outlook 2007 right click on the message select Message Options...

I would check the headers as mentioned above, please see a link of how to do this;

http://office.microsoft.com/en-us/outlook-help/view-e-mail-headers-HA001093707.aspx

I would also check for malicious software on your network

Thanks for that guys, I will look into the headers.

As for malware sophos security centre reports all my machines are clean, are there any more steps I could take on my mail server?

I also scan the server regularly with malware bytes

If the senders appear to be from an internal user, then the Email sender address has been spoofed.
One way to combat this type of spam is to setup a valid SPF record on your domain and make sure you use SPF record checking when you receive mail as this will filter these sorts of messages out.
If you need more info on SPF record setting up, please let me know, but fo rnow, please visit this link:
http://old.openspf.org/wizard.html
You can check your SPF record (if you have one) at:
http://www.mxtoolbox.com/spf.aspx 

Thanks foir taht, i will look into spf & let you know