[Exim] redirecting bounce-messages for one special account

I'd be very happy if someone could give me a hint on this, I've been reading
lots of documentation but didn't figure out how all this filter-stuff works.
I played around by putting filter-rules and a "# Exim filter "-Line in the
.forward-file, put couldn't get it to work. :(

I have a .forward file in one of my home-directories in order to pipe alle
messages in a programm that processes the mail:

|/home/user1/processmymail

There seems to be a bug in that program, I found 5 log entries in the mainlog
(within hundreds of successfull deliveries to user1):

[...]T=address_pipe: Child process of address_pipe transport returned 255
(could mean shell command ended by signal 127 (Unknown signal 127))[...]

I could not reproduce any of the errors, but of course an error-message was
sent back to the sender of the mail.

How do I configure a filter, that all error-messages that are produced by
piping the message to my tool are sent to admin@domain and the message itself
is either sent to a "backup-mailbox" or put into some file? I don't want *any*
bounce-message to be sent to anyone who sent mail to that user1-account.

I am using exim 3.35-1 (debian woody). Never thought about upgrading to exim
v4. Does that make sense or should I update? Would it make solving this
problem easier?

thanks for any help or link...
Jan