[exim] Forcing a message to be deferred if return_path has a…

Hi,

I have a "custom" lookup implimented in perl which is used to set the
return_path variable on the outbound transports. I am wondering what is
the best way to force the message to be deferred if the lookup
encounters a problem? Returning a forced failure just means the message
is delivered with the old return_path, which is undesirable as it is
likely unroutable outside of the local system. The docs say "if it
fails for another reason, delivery is deferred". What would "another
reason" be in this situation?

Thanks,

Gary