[exim] envelope recipient rewriting at transport time

Hi all,

In the documentation Ch 31 "Address rewriting" it says:

    "However, it is not possible to rewrite envelope recipients at transport
    time."

I was wondering if this is a technical restriction or something else? I
would like to rewrite the envelope recipient domain for relay_domains if a certain
condition is met. I do not like the idea of rewriting the address before
transport time as it would require additional configuration on the relaying
server.