John C Klensin writes, answering me:
Amazingly, the sendmail SMTP client didn't dot-stuff by default.
The unix vendor for which both I and the affected recipients
worked had neglected to change this in its sendmail.cf.)
The technical term for this behavior is "bug". 821 is sufficiently
clear on the subject that even the idea of an SMTP sender that can
even be configured to not dot-stuff falls somewhere between "bad
design" and "nonconformance". Now, if the sendmail client you had
was of the vintage I suspect it was, I can imagine why that option
might have been there and even been the default.
I can imagine the reason too. That's part of why I didn't call it a bug.
But I do think that particular feature has outlived its usefulness,
especially since it was an 8.x vintage. I think sendmail 8.12 still has
that default. At least
about 'X' in the same way as I remember.
Now that even the biggest MTA has mud on its face, shall we conclude
that it _is_ possible to get this wrong, and that 2821bis can include
an example or other gentle guidance?