Troubleshoot and resolve TypeError guessFileExtension() errors.
Troubleshoot and resolve Unable to communicate with the WHMCS installation errors.
Troubleshoot and resolve The sender's email address is not registered. (WHMCS 8.4 and later) or Unregistered Email Address (WHMCS 8.3 and earlier) errors.
WHMCS may block email replies from senders who are unregistered, are not CC recipients, or are not associated with the ticket's associated client account. You can review and import these replies into WHMCS manually.