[exim] 451 4.2.1 Mailbox busy, try again later

Hello

I have a remote mailserver which is replying with the following

451 4.2.1 Mailbox busy, try again later

After this step the email goes in my exim mail queue .
I suppose they are using a greylisting/delaying which invite to resend later , right ?

ok , now my chief , asked me that he wants receive an email notification to the error
above "451 4.2.1 Mailbox busy, try again later" from our EXIM/MTA .
While I think is not a good idea , how to do that ?

Thank you
Graziano