Thanks to everyone who responded to my request. And I apologize that this message isn't properly threaded.

I'm in trouble whenever I have to send mail from my gmail account because I automatically pull it down into the same place from which I send my regular mail (from benfell@parts-unknown.org ) and I haven't configured that to deal with the gmail account. That makes it difficult to reply properly to mail on this list which I have subscribed to with the gmail account so that I'd actually receive it even when I'm having problems with my main mail system.

Timo Sirainen <tss@iki.fi> responded that I needed a userdb and offered that I should probably add the following to dovecot.conf :

userdb { driver = passwd }

As near as I can tell--and it's been over a day now--that solved the problem.

Thanks!