Strictly speaking, MIME is not an email format.  It is a mechanism for encoding attachments within an email message.  When you receive an email in what is commonly called HTML Format, you actually receive a MIME-encoded HTML attachment that your email client renders in an internal browser.
Outlook Express does handle HTML S/MIME but it is possible to turn it off.  It may be that this is what has happened.
I would still recommend, like the others, using Thunderbird as your client.  An alternative is Popcorn, which is a very simple lightweight client.  Popcorn is no longer supported but you can easily find it and it is so simple that it needs no support.