> I have to add a "Reply-To" Header in (smtp-) submitted messages.
> Adding it unconditionally using PREPEND result in messages with more
> then one instance
> of this header which violates RFC5322.
> Is there a way to add a header _only_ if not present?
header_checks has no memory, so you need to use a Milter
(Python, Perl, C) or SMTP-based filter.