Going digital - but to what purpose

Yesterday I recieved my Digital certificate from TDC. TDC provides digital certificates for danish citizents. I succesfully installed it in Firefox and Thunderbird but I’m having trouble when sending emails signed with my certificate…. When I recieve them I’m told that the email can have been altered since it was sent - WTF?!?

Dunno if sending emails throug SPAM assassin, virus scan etc. alters the email so the digital signing fails. Well I hope I find some über geek who can help me out :)