Are you using an Exchange server?
If so, launch system manager, Go to Global Settings, Internet Message format.
Now, you have two options here:
1.) Modify the default domain rule
2.) Or create a domain rule to target a domain you know is an Exchange domain
in either case, you want to edit the properties so that on the message
format tab, you have selected UUEncode and on the advanced tab in the
exchange rich-text format tab, you have selected always use.
Caveat:
If you change the default rule to these settings, then foreign messaging
systems that cannot handle exchange rich-text will convert the message to
plain text and all extra formatting and attachments will be rolled into a
"windmail.dat" file on the recipient system, so use this option judiciously.
What I've done in my environment is change my default and then I've created
rules for email domains I know to be HTML based (google, yahoo, etc) and set
mail bound for those domains to force html.