Looks like it's working now. I did this:
aptitude install maildrop=2.0.4-3
Strange. I did an apt-get upgrade the other day, but email was working after that. I didn't do the ispconfig upgrade until the next day and email stopped when I did that.
If anyone has insight into this I'd appreciate it. I've done:
aptitude hold maildrop
to avoid another upgrade, but I don't really like that solution.