[exim] Proxy Error 503. Can change that ?

Hi,

I have two servers, one with Haproxy and its send-proxy experimental feature, and a Baruwa server, with exim installed on it.
Sometimes, I got bunches of proxy 503 errors, due probably to overloads in traffic. They are very annoying for my customers.

Is it possible to change the 503 into a 45x error, just to tell to the others SMTP MTAs to retry later (I cannot compile/recompile exim, it is distributed by Baruwa) ? Or any other solutions to avoid a returning mail to senders ?

Best regards

Sylvain