Maybe this will help.
Your email certificate is used by people to encrypt when sending you an email.
It sounds like you are trying to send an encrypted email to someone whose
certificate you do not have....as you say 'unless I want to mail myself'.
I know it sounds a little backwards, but you can only encrypt mail to someone
whose certificate you have...ie they have sent you a signed email and you have
stored that in your address book.
Hope this makes sense