Rfc 822 is the standard for the internet text message, if you are using MS exchange server and if someone is the org is using the actual Outlook client instead of using thunder bird or web client and decided to use a *pretty* stencils of some other outlook client specific thing then no other email mail client can open it , including the MS web client. 

only 2 solutions for this
1. Find a windows box.
2. use telnet.

Log in using telnet and then select the folder and go through message by message till it complains that it is not rfc 822 formant then delete it.