Anyone who is telling you that you need to pay for ClamXav has no idea what they're talking about. ClamXav is and always has been free. Nor is it likely that the hack of your email account was caused by any malware on your iMac, though it will certainly not hurt to check. You can download ClamXav here:
More likely, though, someone obtained your email credentials elsewhere, if you have confirmed that your email account was indeed hacked.
BTW, you posted in forum for the tower Mac Pro system. Just FYI for future questions.