Can't send emails to custom domains (non-gmail/hotmail emails) using Mailgun - ruby-on-rails

Question

I am sending out confirmation (and other emails) when users sign up. The problem is that when I send to addresses that are not gmail or hotmail or any of the larger email services, it throws me an error in the email header.
{
"severity": "permanent",
"tags": [],
"delivery-status": {
"tls": false,
"mx-host": "mxer.customdomain.com",
"code": 553,
"description": null,
"session-seconds": 6.566637992858887,
"message": "553 5.1.8 <user#customdomain.com>... Domain of sender address bounce+e06614.0f9698-user=customdomain.com#mg.mydomain.com does not exist",
"certificate-verified": false
},
"envelope": {
"transport": "smtp",
"sender": "accounts#mydomain.com",
"sending-ip": "123.45.678.900",
"targets": "user#customdomain.com"
},
"recipient-domain": "customdomain.com",
"id": "AbCDeFGhiJk_lMn",
"campaigns": [],
"reason": "generic",
"user-variables": {},
"flags": {
"is-routed": null,
"is-authenticated": true,
"is-system-test": false,
"is-test-mode": false
},
"log-level": "error",
"timestamp": 1456123374.411238,
"message": {
"headers": {
"to": "First Last <user#customdomain.com>",
"message-id": "56caade72ed26_***********9#6e5120c5-0d27-4491-a430-1a2dc4e6b862.mail",
"from": "Team Accounts <accounts#mydomain.com>",
"subject": "Confirmation instructions"
},
"attachments": [],
"recipients": [
"user#customdomain.com"
],
"size": 1143
},
"recipient": "user#customdomain.com",
"event": "failed"
}
Did I set my headers wrongly or is there a configuration I need to set up before I can send emails to custom domains?

Answer 1

It seems that for some email services, it requires a valid "receive" MX record for your domain/subdomain (for Mailgun it was mg.mydomain.com) to verify the identity of the sender. I added the MX records
10 mxa.mailgun.org
10 mxb.mailgun.org
for mg.mydomain.com and it seemed to have worked.