[Exim] qualify domain question

I am wondering if there is a more elegant way to handle the
following question than constantly manually switching in root
between temporarily commenting out 'qualify domain=msn.com'
and uncommenting it in /etc/exim.conf

I would like to bounce *selected* mail from one local account that
is the recipient of posts received from an external account, to
another local account, say, david, which corresponds to an msn
account and is therefore a bonafide sender of replies to remote
addresses via msn

If qualify domain is uncommented, exim appends the domain and
queues the mail for external delivery. If I comment out 'qualify
domain=msn.com' the mail is delivered correctly locally to david,
but david then exim queues mail from david with the from address,
'david@debian' even if I manually enter 'david@???'

I am using mutt as my mailer and exim as my MTA. I am not
running as a server but use exim to handle BSMTP. Otherwise
the setup generally works quite smoothly

One solution of course is to fix the problem in .fetchmailrc so that
all of the mail is received at 'david' but that would clutter this account
with a lot of mail in which it has no interest

TIA

David