Mail Queue gave this error message: WPMQ Error. The wp_mail() function already exists. This is likely to be caused by another plugin.
I discovered that it was WP Security Scan causing the problem. The error message went away as soon as I disabled Security Scan.
The Mail Queue developer said this, "Having just had a look at the [WP Security Scan] code, it's because this plugin requires the WP core pluggable.php file on loading instead of using the action API. That means the pluggable code is called earlier than it should be."
Link to Forum Post.
Hope you guys fix this soon.