do you have screenshots of these codes? Or a screenshot of your send mail script step in filemaker?
As for "why", the default formatting that filemaker generates for an email is just being handled differently by outlook then the other programs you mentioned.
Possible solutions would be to switch to SMTP sending that bypasses the mail program entirely, switching to an email plugin that supports HTML email, switching to the outlook manipulator plugin that is designed to work with outlook, or figuring out what manual steps are required by your user to tweak the message in outlook before it is sent.
tthank you for your reply. I'll come back to you tomorrow.