Don't terminate with bad argument when passed a non-parsable ARG (gpgit expects an email address).
Instead, return the unencrypted mail aka STDIN.
This gets used in Exim to default to no encryption if no domain to GnuPG key address can be looked up in the router_smtp statement.

  transport_filter = /usr/local/bin/gpgit ${lookup{$domain}wildlsearch{/etc/exim4/gpg-mappings.txt}}