[Exim] relaying for another domain

Hi,

I am using a domainlist router to relay mail for another domain when it's smtp
server is down. I store the mail for that domain in a single file in BSMTP format.

Here is an extract from the exim FAQ :

Large ISPs who provide this kind of functionality do not usually leave waiting
mail on the MTA's queue. Instead, they get it delivered into per-host
directories, one message per file, in one of the special formats (BSMTP,
maildir, or mailstore)

Why does the FAQ say the mail have to be stored in seperate file ?
when I transfer the mails to the other host i think it cost less to transfer a
single file than to open different file.

lets say the domain i relay is domain1.com
when i receive a mail from the outside world for x@???, i store it in a
BSMTP file, whithout even trying to transfer to it's MX, because if the message
when to my smtp server, that means that the primary MX for domain1.com is down
(my smtp server is the second MX)

the problem is that when i send a mail to that domain from my smtp server, it is
stored in that file without trying to contact it's primary MX.

I would like for mail to domain1.com from my host to first try a normal
lookuphost and then if it fails use the BSMTP transport.

how can i set that ?

thank you


Edouard