Email not working for blog…?

I upgraded the blog last night. I was running an old version of WordPress, but now it's chugging away on the latest and greatest, and everything seems to be working. However, I have a reader who has complained that the "Email This Post" link at the bottom of every post has never worked for him. I never had a problem with it, and I have a log that shows that lots of people use it. But just in case, I just upgraded that software. So now, of course, it doesn't work for me. Does it work for anyone else? When I use it, the mail doesn't get sent, and the popup goes to an "Error 404 Not Found" page. I can see from the URL what's going wrong, but the code is a plugin, something written by an independent coder, and it's messy. I can't figure out how to fix it, and the problem is probably with some way I have my permalinks set up. If anyone out there understands this stuff and has a suggestion, I'm listening. I like the idea of readers being able to send a post to a friend, so I'd like to have this working.