500 Error on Email Tracking Page if WP Core in Subdirectory

From includes/smtp.php:

$parse_uri = explode('wp-content', $_SERVER['SCRIPT_FILENAME']);
require_once( $parse_uri[0] . 'wp-load.php' );

This assumes that wp-load.php is in the same directory as wp-content/, which is not always the case. In many Composer-based setups, WordPress itself is in a subdirectory such as /wp/, and thus this require will fail and crash.

This prevents such sites from having any way of getting rid of this rather obnoxious advertisement at the top of every admin page (it really should be limited to just the plugin’s own settings page).