Switch from an external Mail Provider to PHP Mail
After updating to WHMCS 8.6.1, you may be unable to select PHP Mail for Mail Provider in the Mail tab at Configuration > System Settings > General Settings.
We have created a hotfix for WHMCS 8.6.1 to resolve the error:
Run the following command to reset the Mail Provider setting in the WHMCS database:
UPDATE tblconfiguration SET value='' WHERE setting='MailConfig';
Then, reload the Mail tab at Configuration > System Settings > General Settings and reconfigure your email settings.