A number of our clients use email only to contact us for support. They never visit the ticket area of our support site.
These clients tend to CC other people in their organisation when they request support or CC our support email address.
We would like it that every address in the email TO and CC header is added to the ticket, then when we reply the reply is sent to all the people in the original email. Much the same way a 'Reply-to-all' works on regular email.
In terms of viewing this ticket and all replies via the ticket area in WHMCS a link in each reply is all that's needed. If any of the addresses in the original email are client addresses then it could be added to their account history as well.