Thanks for diagnosing this. It is indeed an incompatibility between
IETFHDR_SUBJECT_TAG and SUBJECTHDR_SEQUENCE. IETFHDR was introduced many
years ago to please subscribers who wanted the mail they got from LISTSERV
lists to look like what they would get from a sendmail /etc/aliases entry.
LISTSERV goes out of its way not to change any existing mail headers,
including the subject, since it would be blasphemy in the eyes of the
beneficiaries. Everyone was happy for many years, but eventually the people
in question changed their mind and requested the IETFHDR_SUBJECT_TAG option.
Not having forgotten the violent debates, this was cautiously implemented by
taking the pristine original subject and surgically inserting the tag with a
minimum of interference with surrounding header tissue. Everyone was happy
again until SUBJECTHDR_SEQUENCE. This is a more delicate edit (because of
the risk of incrementing the sequence number when the message is in fact not
distributed, or incrementing it twice). To make it work, I will have to use
the normal, edited subject for IETFHDR_SUBJECT_TAG. If anyone complains, I
will point out that it is clearly better than the current behaviour :-)

  Eric