Cannot Select PHP Mail as Mail Provider
Symptom
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.
Solution
Update to the WHMCS 8.7 release at Utilities > Update WHMCS.
Hotfix
We have created a hotfix for WHMCS 8.6.1 to resolve the error:
https://whmcs.community/files/file/185-core-18329-switch-from-an-external-mail-provider-to-php-mail/
Workaround
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.