All- I noticed that GMail likes to offer to "Invite to GMail" every time I post to one of these mailing lists... I've almost invited on several occasions, and I'm sure that spam would have been annoying.
If you want, the following line will add a custom filter for AdBlockPlus users in Chrome and Firefox to prevent this from showing up; you may need to restart the browser, though: mail.google.com##span#link_invite -- Christopher L Tubbs II http://gravatar.com/ctubbsii
