[exim] accessing the error code for an email

Esteemed Exim users,

I'm writing a Perl script to take action on SMTP response codes. Where
is the best point to intercept in coming email with its SMTP response
code?

I can get the email message from a .forward file as it is deposited,
but I'd like to access the information represented in the message's
line in the mainlog as well:

2004-08-24 11:51:05 1BzgNr-000LZQ-0D ** bla@??? R=dnslookup
T=remote_smtp: SMTP error from remote mailer after RCPT
TO:<bla@???>: host mx2.umich.edu [141.213.231.71]: 550 Requested
action not taken: User not found.

Is this available at the time of delivery to the ~users/.forward
somehow? Or perhaps I am better off intercepting the bounces before
the inbox. If so, where?

Sincerest thanks for your expert advice,

Austin