exim4 customizing for only sending systems

Hi, I have some problems of understanding / configurating exim for some special advantages. In my systems are some clients with software, witch is not able to receive mails. For each client(8) I have different send and recivie adresses (16). So I changed the envelope sender. Does exim send errors like exceeding the mail-size-limit or undelivery to the new envelope-sender or is the address-change later applied? Should I use the reply-to header (they say it is deprecated)? And I want to set the new envelope-sender as errors_reply_to-address. Thanks in advance.