Mail Virus? Why does Mail get 300 spam messages a day and the provider mailbox has 0 of these emails.

You've apparently gotten onto spam lists, or someone who had your email address in their address book has contracted an email virus or bot malware that is allowing the spammers to send to your address. There's nothing you can do other than filter out the spam or change your email address.

Regards.

And, unfortunately, many times the removal links just act as verifications to the spammers that your e-mail address is valid.

All of your devices are connecting to the same email account through which this spam is arriving? If so, is it a POP account, IMAP or Exchange?

Any chance that spam filters on the other devices are trapping the junk before you see it but that your Pro isn't? And there are no other email address, such as iCloud, configured on your Pro?

There have never been any confirmed viruses nor worms that can infect Mac OS X, and none of the trojan horse programs that I've seen reported have this sort of behavior. But I can't say it's impossible. And that you've erased the drive and reinstalled but the problem persists would seem to confirm that it's not malware. But what is causing this just on the Pro I don't know, assuming again that you don't have iCloud or some other email service set up on the Pro that's not on the other systems.

I'm sort of out of ideas right now. Perhaps someone else will have some thoughts on the matter.

Regards.

Is there a way to see the email headers?

Select a message, and from View menu -> Message choose All Headers or Raw Source.

Some people have reported success using SpamSieve in addition to Mail's junk filter. There is a free trail period.

Spam - SpamSieve