If someone tests this and their email goes to spam, would they be so kind to pastebin me the message headers/mime so I can try to solve that together with the Mailgun folks? Thanks a bunch.
http://litmus.com has a free trial with tools that might be useful for you.
How about creating a test email a\c by yourself to see if the email goes to spam or not? Your existing email a\c may also work.
It doesn't go to spam in my Gmail, which it did yesterday. However, some people are reporting the mail going to spam. This is why I'm asking, I can't test it myself on any account.
Another good test is http://isnotspam.com/. By sending an email from your app to check@isnotspam.com, you can get some good feedback on spam triggers.
I like http://www.mail-tester.com/ -- that creates a unique address just for you, while isnotspam lets you search for anyone else's message.