Problems making local postings to inn2

[Jeffery Small]

I recently upgraded from Ubuntu 16.10 to 17.04 and have been having many
problems making local postings to the news system.  There was likely
an upgrade of inn2 to version 2.6.1-1 at this time.

For years I've been using the mailagent(1) program to filter various incoming
emails into the news service and posting still works in general, but I am now
seeing a LOT of the following new messages in my mailer log file:

    inews: cannot send article to server: 441 Invalid syntax encountered \
    in headers (unexpected byte, no colon-space, or empty content line)
    inews: article not posted

Occasionally the messages refer specifically to the References: or Subject:
header lines.  For example:

    inews: cannot send article to server: 441 Invalid syntax encountered \
    in References: header field body (unexpected byte or empty content line)
    inews: article not posted

    inews: cannot send article to server: 441 Invalid syntax encountered \
    in Subject: header field body (unexpected byte or empty content line)
    inews: article not posted

These messages get left in my inbox and I have investigated the headers of
these failed posts in great detail.  I can find no instances of:

    * A header that does not have a "Name:<space>" format
    * A header with an empty field after the :<space>
    * Any odd characters: e.g., no non-ascii-7 characters

Everything appears normal, so I am at a loss to see why these fail.
Any suggestions as to where to look for the problem?

Also, is there a way to get inn2 to generate better diagnostic output.
Despite all these (and other) problems, the log file /var/log/news/errlog
is always empty and the other news logs have little useful information
to help isolate a problem.

Thanks.
--
Jeff Small