I think the theory that the source of the email is spammers posting in threads which you've subscribed is correct. Occasionally spammers will post jibberish and links. The mods are quick to remove them.
It looks like you have instant notifications set up on some threads which would mean as soon as a new post is made it is emailed to you. So even with mods removing the spam, the email is still sent. There are 3 email subscription options: Instant, Daily Digests, Weekly Digests, which indicates the frequency that you'll receive an email when there are new posts. Subscribed threads will continue to send an email for as long as there are new posts.
You can manage and change your thread subscriptions by clicking on the Quick Links link which appears just under the "Welcome, cawhouston" text that appears near the top right of any of the forum pages. From the menu that appears, click on Subscribed Threads. Here you will see all the threads you are subscribed to and their current subscription mode.
I hope this helps.