The Mailgun Plugin for WordPress needs to be updated. https://wordpress.org/plugins/mailgun/ I am a fan of MailGun and want to see you all succeed. Currently there is a warning banner on the WordPress plugin repository saying that the plugin has not been tested with the last 3 major releases of WordPress. This plugin currently has over 90,000 installs - I'm sure many are paying customers.

Also, it would be great to integrate error reporting into the plugin so WordPress is notified when an email fails to send. This way other plugins, such as WP Mail Log, can pick up and report on that error.