Before version 8, if an email came in with a ticket number in the subject line that reply would be added to the thread. This would be the case for situations such as a distribution group being CC'ed on a ticket and one of the members of that group replying. Before version 8 this worked exactly as expected, now the ticket import fails whenever the sender is not already in WHMCS. This is an absolute deal breaking, find a new platform level problem for us since emails from clients are getting missed unless we're constantly monitoring the ticket import log.
Thanks for your suggestion and votes. I'm pleased to confirm that a new "Allow Insecure Imports" option has been added in v8.3 which does this: https://docs.whmcs.com/Support_Tab#Allow_Insecure_Imports