Problems decrypting Comodo freeware email

I have downloaded the Comodo Email freeware and have tested it. However, I have a bit of a problem. I can send encrypted mail TO myself and TO my son, who also downloaded the freeware to help me get started. I can decrypt mail from myself, but I cannot decrypt mail from my son. What am I missing?
My system = E-Machines- WinXP-Outlook Exp.

I really like the Comodo Email system (over and above PGP) since it seems to be much more user friendly, at least to a novice like me.
My only complaint (other than getting it to work :SMLR, is finding a good source of instructions for using it. There must be more to it than the 2 radio buttons it installed on my tool bar. Is PGP more complicated because their software has more bells and whistles?

Thanks for any help,