Backstory

I have email via my own domain name, and lately have been increasingly inundated by SPAM sent to random email accounts that I have never even registered or "released into the wild" on the internet. Besides the obvious irritation of having to download a thousand emails when only 5 are legitimate, I often have problems with my local email client, Thunderbird, failing when it is trying to download 1500+ email messages. I'm not sure if it's Thunderbird or my hosting provider who's to blame. But I digress.

Anyway, my host offers SPAM filtering for a monthly fee, but it irks me to have to pay money as a result of unethical spammers. So, it recently dawned on me (not sure why I didn't think of it before now) that I could utilize Gmail's POP feature in conjunction with their external accounts feature to construct a SPAM-filtering "router" for the email I download via my local email client.

High Accuracy SPAM Filter

Considering the enormous amount of SPAM that naturally ends up on Gmail's servers, that huge "sampling" of user-identified SPAM is used to feed Gmail's SPAM algorithms, constantly tweaking them and therefore bolstering the algorithm's high accuracy rate. So, not only do I get a free SPAM filter, I get one of the top SPAM filters around. Candy. (No, Candy is not the name of Gmail's SPAM engine, it's an expression, like "Sweet".)

How to set up Gmail as a SPAM-filtering "router"

The way to set up Gmail as a SPAM-filtering router is fairly easy. Register or log in to an existing Gmail account. Then go to Settings > Accounts > Get mail from other accounts, click "Add another mail account" and follow the steps to do just that.

Notes:
1) You are limited to only 5 external email accounts per Gmail account.
2) If you have a lot of emails already waiting to be downloaded from the external account, Gmail will start downloading from the account at a maximum rate of 200 emails at a time, at inconsistent intervals (presumably to distribute server/bandwidth load).
3) Gmail will download email from external accounts even when you are not logged into Gmail.

Once the external email account is set up, go to Settings > Forwarding and POP/IMAP > Pop Download and chose to enable POP. Then click on the "Configuration Instructions" link for help on configuring your particular email client to download email from Gmail's server.

Note: As you probably have noticed, you can also use IMAP instead of POP if you prefer that method and, of course, understand the difference between these two email download methods.