This is a problem we've had too. Up until the SP1 update, unchecking the HTML format preference allowed plain text formatting, which worked. But the SP1 update for Outlook caused a problem, because even having the HTML format preference unchecked didn't help. A quick workaround is to either use Mac's Mail program to send your emails with, or when you use Outlook and want to send a message, once your message pops up to send, use Shift+Command+T and Outlook will convert your message into plain text, which then recognizes the carriage returns.
But if you are sending out mass emails, this is a problem. I've posted my concerns to Microsoft, but as of yet, no info on what they plan on doing.
So far, a MS representative has said, "Outlook supports format=flowed now and I don't think you can turn this off. I'm checking and will get back if I hear something."
To our company, this is a major problem. If it's not resolved, we are going to look for another email program.
I have just upgraded to the new Mac Outlook Microsoft suite and after many hours of frustrating carriage returns not being recognised I discovered your post.
A work around is to replace the Filemaker's paragraph return symbol for the HTML code "<br/>".
Well, the moment it works for me and we back in business.