$=s
Presume an RFC2822 7-bit body V8.7 and later
An email message as defined by RFC822 cannot contain
8-bit data. Consequently, when the MIME Content-Type: header
declares a message subtype that is rfc822, we immediately
know that it will contain nothing that needs 8- to
7-bit encoding:
Content-Type: message/rfc822
As other message subtypes evolve, this assumption can
safely be made about them too. So, to make
sendmail more adaptable, the
$=s class was
added beginning with V8.7. This class contains a
list of subtypes that should be treated the same as
rfc822. When
sendmail first begins to run,
it initializes that list to contain:
rfc822
Other subtypes that can legitimately appear here might
be partial or
delivery-status.
Note that this provides only an initial hint to
sendmail. The rfc822 subtype can
itself contain MIME information that might require
8- to 7-bit encoding.
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