[exim] Help with problem with bad bcc addess

Debian package exim4 version 4.50-8.

One server (Server A) uses another server (Server B) as a smarthost,
which sends the mail onto an Exchange server Server C. Another Server
D sends email directly over the Internet. The application is a Java
application running on Linux and runs on yet another server..

When there is an invalid address in the bcc field of the email (as
well as some valid addresses) then the two systems, Server A and
Server D act differently. By 'invalid address' I mean one of the form
someone@???, where domain.com does not exist.

Server A gets a message that Server B cannot relay for the invalid
address, and NONE of the addresses in the message receive an email.

Server D gets a message that the invalid address is Unrouteable *but*
delivers the message to all the other, valid, addresses.

Obviously the way to solve this is to stop the application from
generating these invalid addresses, but I'd like to understand why the
difference. Why, when the email is sent to a machine that cannot relay
that single address does it refuse the whole email, yet when delivery
over the Internet is attempted it delivers to the valid addresses. OK,
two different processes, I know, but I'm not understanding something
about the processes, obviously.

If there is a FAQ along these lines, I've not found it. If it's in the
documentation I don't know the correct place to look, though I have
looked. If it falls into the "bleeding obvious" category, I'm sorry,
I'm not sufficiently familiar with exim4 yet to see it. Any help and
insights will be appreciated.

Cheers,

Cliff