There's fairly extensive help available from the list 'bot itself.
Try sending a message with
help
help set
to majordomo(at)postgresql(dot)org(dot) (There are a bunch of other help topics
but I'm guessing "set" is most likely the command you need.)
A low-tech solution would be to subscribe all your addresses and then
set all but one to "nomail". Not sure if there's a better way.
regards, tom lane