Anyone having issues sending to addresses using mailgun? <SOLVED>

I switched to mailgun after mandrell went away as suggested in discourse. However, now all of my emails sent to email addresses are failing with: “550 5.1.0 Invalid sender domain”. It works for every non comcast customer though.

This obviously isn’t an issue with discourse, but I was hoping someone might have some insight. I called comcast and they were completely useless and have opened a ticket with Rackspace. We will see what they say. In the meantime…

Thanks for any help!

I figured it out. I thought I would document it here in case anyone else needs help in the future.

The sender of the email was coming across at comcast as ‘’, which comcast was trying to verify.

I needed an additional MX record. I use Google Apps for normal emails, but also Mailgun. I had all the Google Apps MX records, but what I had to do was add an additional MX record with a subdomain of “mg” for mailgun, which I just pointed at google server. Here are my MX records, which seem to now be working:

MX Records
Mail Server	Preference	Subdomain	TTL	1		Default	5		Default	5		Default	10		Default	10		Default	20	mg	Default