gnus-article-loose-mime

gnus-article-loose-mime is a variable defined in `gnus-sum.el'.
Its value is

t

Documentation:
If non-nil, don't require MIME-Version header.
Some brain-damaged MUA/MTA, e.g. Lotus Domino 5.0.6 clients, does not
supply the MIME-Version header or deliberately strip it from the mail.
If non-nil (the default), Gnus will treat some articles as MIME
even if the MIME-Version header is missing.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.