Blacklist senders & domains
Ability to blacklist a specific sender or a specific domain to avoid Mailgun to send emails to these. In this way one can avoid sending a "successfully received" email as replies to Twitter, Facebook and LinkedIn invites, plus to unsolicited senders (not necessarily spam, but unsolicited in the eyes of the Mailgun user).
Travis Swientek commented
Bart - You could do this within your server side code. Just check the recipient address. If it contains Facebook, Twitter or LinkedIn, don't send a message.