A passcode is a word, phrase, or group of characters used to identify an e-mail as non-spam. It may appear in the subject line, but does not necessarily have to.
I use passcodes with my e-mail. I get a lot of spam, about 6-8 spams for every real e-mail. As a result, I implemented my own spam checker. It does a great job, though there is a small possibility that a real message could end up labelled as spam. The passcodes are a way to prevent this.
One reason that an e-mail might be classified as spam: if there is a mismatch between your address in the two "from" lines, then the message will be labelled as spam. This can occur with mailing lists; the first "from" indicates the mailing list, while the second "from" indicates the person who originated the message. If you sign me (or anyone else) up for a new mailing list, you should know that it is bad netiquette to sign someone up for a mailing list without his/her permission. Speaking of e-mail etiquette, I also recommend reading how to write an e-mail.
Of course the people who regularly e-mail me, and mailing lists that I subscribe to, are both included on my "white hat" list. This means that their messages will be classified as "good" automatically. This list only includes accounts that I know; for example, if I normally get an email from you as email@example.com, then you open up a new account as firstname.lastname@example.org, then I won't automatically know that the second account belongs to you.
So, if you've read this far, you've never sent me an e-mail before, and you want to send me one, use the word "gopanthers" in your subject line. I may change it in the future, especially if a spammer starts using it. If so, I'll change it here, too.-Michael Weeks