@SendGrid, all our clients are using PHP 5.5, MySQL 5.6, and the latest version of WordPress. It's possible given the upgrade to WooCommerce 2.5 in the past 48 hours that it could have been related; it does seem like one possible correlation among our various clients, but the list of plugins they each have installed varies greatly.
If possible, I would encourage testing your plugin extensively on a variety of different WordPress setups before releasing updates, and especially checking for compatibility with the latest versions of WooCommerce because there are several large eCommerce companies using your plugin on our network that were without email notifications for several hours due to this hiccup >_<
Anyway thanks for your quick reply and for now it seems the Postman SMTP plugin is working very smoothly with your API. Thanks again ~