RFC 1652 specifies two parameters for the MAIL FROM command: "7BIT" and
"8BITMIME". 7BIT indicates compliance with RFC 821 whereas 8BITMIME
indicates compliance with RFC 1521 "with arbitrary octet content".
Why is there no "7BITMIME"?
Because there is no need for such a thing. The transport for a regular 7bit
RFC822 message is exactly the same as the transport for a 7bit MIME message,
since 7bit MIME messages are in fact a proper subset of 7bit RFC822 messages.
When sending MIME messages that are strictly printable us-ascii encoded (i.e.
no 8BIT or BINARY) how should a mail client check if the mail server is
capable of interpreting 7-bit MIME? Should the client simply check for
8BITMIME support?
Mail servers don't interpret the content of messages so this is an entirely
meanless question to ask. Only the user agent that displays the message to the
user is involved in interpreting MIME. The specific capabilities change
depending on the user agent, and most systems have a variety of user agents
available, each with differing capbilities. New agents can also be added
containing new facilities and these facilities can be applied to previosly
received message. As such, even if a means existed to query a mail server as to
the capabilities of the clients that use it, there is no way for it to give a
meaningful yes or no answer to such a question.
Ned