[dba-Tech] certs and e-mail

I don't understand how cert id's are supposed to work with E-mail. My husband has one at work and although I can read his mail and he can read mine, I can't respond to his mail. I get an error that says I don't have the cert id, but it offers an option to get it, when I try to get it, I get another error. Is this a problem on his end or mine? 

I'm not sure I understand the cert's purpose with e-mail. 

Susan H.