Carl Malamud writes, answering me:
I'd say that ONLY the first MTA may add SOLICIT=, and ONLY based on
the Solicitation field and/or the MUA's SMTP/SUBMIT SOLICIT=
argument. Other MTAs may not: if there are Received fields at all,
neither they nor Solicitation may be parsed to construct SOLICIT=.
Hmmm ... I'd simplify that one more time. You need to be able to grab
the solicitation header in case there are non-supporting mtas in the
I considered that and IMO, that's not necessary. MTAs that are neither
under control of the sender nor teh recipient generally only occur for
mailing lists, and noone who's honest enough to use SOLICIT should be
spamming a mailing list.
Btw, I also suggest sticking in some text noting that supporting SOLICIT
as server does not require supporting it as SMTP client. For ten years,
people have been assuming that supporting PIPELINING is hard, even
though it's dead easy for the server.