take 2 -- anyone? how to dovecot-LDA 'deliver' to address+delimiter@mydomain.com?

hi all,

i've exim 4.53RC2 + dovecot-cvs on OSX 10.4.2.

i'm working on implementing recipient-delimiters for delivery from exim to a user's imap store using dovecot's cvs-lda.

reading on the wiki (Jakob's latest comments?):

" ... the part after the delimiter is passed to the lda as an additional parameter. so dovecot-lda should handle that parameter and leave the separator handling to the MTA. The MTA has to handle it anyway."

it looks likes it's possible ...

with appropriate changes in exim to my router:

dovecot_localuser:
    driver                  = accept
    domains                 = +local_domains
    local_part_suffix       = +*
    local_part_suffix_optional
    transport               = dovecot_lda

and transport:

dovecot_lda:
    driver                  = pipe
    command                 = /.../deliver \
                              -c /.../dovecot-deliver.conf \
                              -d $local_part@$domain
    message_prefix          = ""
    message_suffix          = ""
    delivery_date_add
    envelope_to_add
    return_path_add
    log_output
    user                    = MY_MAIL_USER

i'd like to be able to build delivery conditions in dovecot so that, e.g.

mail to:    bob@mydomain.com
is delivered to "bob"'s default INBOX.

but,

mail to:    bob+spam@mydomain.com
is delivered to any specified (either a server-wide common relative

path, or a user-specified) SUBFOLDER, say "bob/INBOX/.../.../spam_to_look_at" defined by a dovecot-rule invoked by the delimiter "spam" ...

this is a little different than the approach Mark mentioned of delivering directly to folder_name specified by the local-part-after-delimiter ...

questions:

(1) until the two conf files merge, i'm guessing there needs to be some conditional logic in dovecot-deliver.conf rather than in dovecot.conf.

correct? or should i be thinking SIEVE rules?

(2) exactly WHAT "additional part" do i grab in dovecot?

thx!

richard