Encryption Problems error Outlook 2007


I have win xp pro and outlook 2007 with comodo free certificate installed. when I send an email i get an error pop up “The Microsoft office outlook had problems encrypting this message because the following recipients had missing or invalid certificates” the option is then! “continue will encrypt the message but the listed recipients may not be able to read it”

the continue option is grayed out? i have no option but to send unencrypted?

Is there a way to manually send certificates to a recipient or group and be able to manage who has installed my certificates?

if I purchase an email certificate will this resolve the problem?