Challenge Response Filters
But I must say that I hate these challenge response filters. Often when my mail gets bounced back for a challenge, I just ignore it or delete it and move on. If the recipient thinks their time is more important than mine, then screw them.
And then there’s the fact that I do a fair amount of email offline (planes, trains, hotels, etc). There’s no way to do the response offline, so it goes into the deleted folder.
I sure hope this isn’t the future of email.