MIME is simply a way of sending something other than plain text by e-mail.
There is nothing wrong with MIME, but if the recipient doesn't have a MIME compatible e-mail reader, they tend to get a load of illegible gibberish, if you send them MIME e-mail. Additionally, HTML based MIME e-mail tends to be about 3-4x the size of plain text (especially when produced by Microsoft's sloppy HTML encoder). For mailing lists, MIME e-mail is undersirable because not everyone can read it and because it takes longer to download.
Rather annoyingly, outlook express has HTML enabled by default. To disable it go to Tools->Options->Send. Under mail sending options, select 'plain text', then click 'plain text settings' and select MIME encoding: None.
Another annoying feature of outlook express is that it has an option to use MIME in news postings (fortunately not enabled by default). Very few newsreaders other thean OE, support MIME in newsgroups (mainly because the internet standards which allow different programs to get along, state than MIME should not be used in news).