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.