If you receive an error message like "The selected domain was not valid"
Edit the /etc/sentora/panel/modules/mailboxes/code/controller.ext.php file and change:
if(!self::IsValidDomain($domain))
to
if(self::IsValidDomain($domain))
Edit the /etc/sentora/panel/modules/mailboxes/code/controller.ext.php file and change:
if(!self::IsValidDomain($domain))
to
if(self::IsValidDomain($domain))