Does anyone know of, or where I can find, a way of testing what logterse is failing to resolve?
Logterse is just a plugin for qpsmtpd, and just "logs" information passed by qpsmtpd, so you really need to ask:
Does anyone know of, or where I can find, a way of testing what qpsmtpd is failing to resolve?
and in particular it is probably another plugin for qpsmtpd (which really only provides a framework) that you need to interrogate.
I suspect (but someone else ought to confirm), that it is a reverse DNS failure we are looking at here.