So you are using static userdb? I'm not sure how this could be fixed. Especially if you're using passdb PAM + userdb static it could be impossible for Dovecot to verify if the user exists (I don't think PAM_USER_UNKNOWN reply can be relied on, plus it could do 2 second delays for each user "lookup" because the password is wrong). With other passdbs it would be possible, but that's again userdb static specific so I don't think I'll bother..
But yes, I could mention this in LDA's wiki page.
Yes. I'm using ldap passdb + static userdb.
I fixed this in exim: If the address is joe+spam@abc.com, I use exim's appendfile transport (and no indexing is done). If the address is joe@abc.com, I use dovecot's LDA.
This is probably even better as very few ppl need their other folders pre-indexed anyways... mainly the SPAM folder.
Thanks