Turns out that the email setup here at Mary Anne and Kevin's place is such that I'm not going to be able to send outgoing mail. I can still receive mail just fine; I just can't send it.
(As someone who travels a fair bit, I'm finding it really frustrating that ISPs are putting more and more restrictions on this stuff. In my world, it's common for someone to travel with a laptop and want to be able to send mail from it using a friend's Internet connection; but ISPs are apparently doing things like (geek alert) blocking access to port 25 on any server other than theirs, and requiring the customer to specify a set of valid email return addresses that they can send from, effectively disabling outgoing mail for anyone other than the customer. I suppose it makes sense from the ISP's point of view, but it makes it hard for people to allow their visitors to send email.)
(Even more geeky aside: I have a mac.com trial account, and mac.com allows you to connect to port 587 if your ISP blocks outgoing connections to port 25. Unfortunately, I can't figure out how to convince mac.com to send my mail—it keeps resetting the connection.)
So for the next few days, I'll only be sending mail during times when I go down the block to a cafe that provides wireless. So don't expect timely responses from me to email. And autoresponses to SH submissions will once again be a little slow. :( But should still be within a day or two. If I keep running into this sort of problem, I may have to reconsider my approach to autoresponses.