[Exim] Delivering one mail at a time

I have a *very* slow program that I pipe mails for a certain local user
to, which will end up rejecting any mails after 10 attempts to lock the
files it's writing to (as another instance of the program is still
delivering an earlier mail).

In order to get this to work properly, I think that what I need is to
encourage exim to only deliver one mail at a time locally.

smtp_accept_queue would do what I'm after as far as mail getting to the
host concerned goes, but once the mail is in the queue then the next
queue runner will deliver all mail on the queue and cause most of the
queued mail to be bounced.

I can't find a similar option for local deliveries by grepping - does
anyone know if this is possible by tweaking some knob within exim ?

Thanks,

Ceri

--
keep a mild groove on