I had a bunch of problems with this myself with plain text e-mails from Linux systems (just text files shoved out using /bin/mail). I did bunches of translation and tried every line ending I could to get what I wanted. I think I eventually stumbled across Line breaks are removed in posts made in plain text format in Outlook.
I don't know if that will help (especially since you are using MIME e-mails), but it's worth a look.
Are you posting in the right place? Check out Where do I post X? to know for sure.
Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:
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Want more info? How to link
or How to display code and escape characters
are good places to start.