recipient bcc and dovecot

[@lbutlr]

I have a recipient_bcc_maps which contains a bcc map that is updated everyday:

rbcc.pcre:
if !/backup.*@/
/^([^+_]*).*@([^.]*)/   backup+071.${1}-${2}@adomain.tld 
endif

the 071 portion is changed each day to the current day of the year.

Everything works, but I get an error from dovecot on every message:

lda(backup at adomain.tld): Error: User initialization failed: Namespace '': Ambiguous mail location setting, don't know what to do with it: /usr/local/virtual/backup at adomain.tld (try prefixing it with mbox: or maildir:)

The mail is delivered two the right directory anyway, but after a temporary failure.

I'd like to eliminate the error.

the backup user is not a 'real' user (it is not in the lookup map) so should I bypass dovecot entirely in delivering these messages? (and if so, how)

If not, where/how would I add the prefix maildir: 

Is there something I can add to dovecot to specifically deal with messages coming in to backup at adomain.tld and add the maildir: specification there?

(Still on dovecot 2.2.35)

-- 
And sometimes there's a short cut. A door or a gate. Some standing
stones. A tree cleft by lightning, a filing cabinet. Maybe just a spot
on some moorland somewhere... A place where THERE is very nearly HERE...
If some people knew where such a spot was, if they had experience of
what happens when here and there become entangled, then they might - if
they knew how - mark such a spot with certain stones. In the hope that
enough daft buggers would take it as a warning and keep away. (Lords and
Ladies)