The sender needs to set your contact to RTF - He needs to open your contact, double click on the address and change it to use RTF.
See
http://www.slipstick.com/problems/alwaysrtf.asp (its for the opposite problen - you want to allow RTF for tasks to work right) - its the 2nd (and additional) screenshot on the right.
Leave the global setting to HTML (first screenshot) - override it with the contact setting.