One-time verification to stop spam on ticket system
I believe this is simple and good solution for eliminate 100% of ticket spam:
1) whmcs receive new ticket
2) if the email come from a client, ticket bypass verification
3) if the email come from a unknown address, whmcs send a message back, asking to click on a link to verify if this is not a spam
4) if the link is clicked the ticket is accepted and the EMAIL is saved on a database table, so in the future is not necessary a new verification from the same email address.
With this, only messages from registered clients and click-verified emails will be accepted as tickets.
The verification only occur one time for unknown email address. Your customer with a non-registered email address wil perform the verification only one time.
This feature should be OPTIONAL on ticket system, and have a tool to delete old emails on database, example: delete all verified emails that not open a ticket on last 1 year - with this tool the table with "verified emails" will not grow forever.
As OPTIONAL feature on WHMCS.