How to get the email address from a client certificate and send it through the header in an iRule.

The email field is in subject and/or extensions - I'd try logging values from the X509 commands until you find it, then you can adjust your iRule accordingly (you'll find the logged values in /var/log/ltm);-

when CLIENTSSL_CLIENTCERT {
    log local0. "Subject: [X509::subject [SSL::cert 0]]"
    foreach item [split [X509::extensions [SSL::cert 0]] \n] {
        log local0. "$item"
    }
}