Neat! Shamelessly, I'd like to mention that you may also consider using the email (https://github.com/jordan-wright/email) package I wrote to serve your backend. I've worked to make it as simple and robust as possible. However, if you're happy with gophermail, by all means use it! Both are great packages, imo.
I appreciate you giving it a shot. If the issue is with my package, please don't hesitate to let me know and I'll jump on it ASAP. Happy to help any way I can!