It’s hard to tell in the real world right now, because another part of the whole system is causing problems, but it looks OK. The old Clickatell server, which is the system which is picky about text encoding, says it processed the email with no problems.
The relevant part of the Thunderbird messages (which worked yesterday) is
Content-Transfer-Encoding: 7bit
The corresponding part of the swithmail messages is
Content-Transfer-Encoding: quoted-printable
Thanks again.
C.