On Tue, 22 Sep 1998, Mark Crispin wrote:
Historical retrospective: MIME recycled the Content-Type header from an
earlier multimedia mail specification instead of using a unique mechanism.
To distinguish between the new and the old mechanisms, the MIME-Version
header was invented.
Consequently, a message with a Content-Type header but not a MIME-Version
header is not MIME.
That's entirely correct and fully compliant.
However, in practice I'd recommend treating any Content-Type header with a
"type/subtype" syntax as a MIME message. The old version of the
Content-Type header didn't have subtypes so it is easily distinguished
from MIME Content-Type headers.