[exim] Advice needed on Bounce messages and backup MX

Hi all,

I have browsed Google and Exim documentation but haven't found a
solution, so I hope someone can guide me:

Setup:
Two servers on seperate domains acting as primary MX as well as backup
MX for each other. (Friends server and my server).

Problem:
My server is rejecting mails detected as spam (sa-exim). When spam is
delivered through backup-MX, that backup-MX is generating bounce
messages, due to rejected delivery on my server :-(

Idea for a solution:
Let my server accept any mail from backup-MX, still scanning for spam,
but instead of rejecting mail, then just pipe /dev/null.

I see two places to patch:
1) Exim-caused rejections, like unknown recepient. Should only reject if
sending host is not the backup MX, otherwise just accept (but same
internal behaviour).
2) Exim-caused rejection. Same behaviour as above.

Does this sound as a reasonable solution (given that the configuration
of the backup MX is not to be touched)?

If so, what config changes is necessary?

~Per