[exim] Custom bounce messages for certain users?

Hello all,

Just a quick one - is it possible to set up exim to send special error emails
for certain users - such as one user at usera@domain find that ppl always email
her at userb@domain and would like exim to send emails to ppl who sent emails to
userb@domain saying "Incorrect address, please use usera@domain instead"?

Thanks very much for your help in advance!

Cheers - Piers