Summary of problem resolution: Outlook Web Access appears to be sending plain text messages in base64 format, which is valid but not a good idea. Because 1.8d did not forward submitted messages in MIME format, they display as garbage, but should be distributed correctly if approved (not verified). In 1.8e the submitted messages are forwarded to the editor as an attachment and it should be possible to open them normally (not verified but works with eg Word). Obviously this does not apply if you use the NOMIME option to revert to 1.8d behaviour.
Eric