Out of the box, only one client email template is supported per table irrespective of the conditions set . "reply-received" and "replyall-received" client templates have (user_id),(user_id, copied) in the "To" field of recipients section by default. This will serve the purpose of sending emails from client templates for internal users only.
For external users user_id will not work. We have to use "user" in the "To" field if we have to send emails to external users.
To handle the case of sending mails to both internal and external users in a single email template(as more than one email templates per table is not supported) ,we have to give the below in the "To" field of the recipients section in "reply-received" and "replyall-received email client templates
"user_id,user" for "reply-received"
"user_id,user,copied" for "replyall-received".
PFA screen shots for reference.