Doh. I am so stupid.
I finally looked at the docs for the pipe transport, and there is the
answer staring at me.
===========================
message_suffix Use: pipe Type: string† Default: see below
The string specified here is expanded and output at the end of every message. The default is unset if use_bsmtp is set. Otherwise it is a single newline. The suffix can be suppressed by setting
message_suffix=
===========================
--
Cheers,
Jeremy