Hi Crispin,
Thanks for this suggestion.

At present support tickets can be opened via email from email addresses which are not associated with an account, user or contact, provided the support department has the "Clients Only" option disabled.
For Client Only departments, the email address must be known to WHMCS beforehand in order for the ticket to be created, so that the message can be associated with the requestor with confidence.

A potential issue I see with the suggestion is; what about accounts registered with mass email providers, like @gmail.com, @hotmail.com?
How would we determine which account to associate the ticket to, in the event more than one client account exists with an @gmail.com address?