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

hosting
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I switched to mailgun after mandrell went away as suggested in discourse. However, now all of my emails sent to comcast.net 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!

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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 ‘xx@mg.mydomain.com’, 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	
aspmx.l.google.com	1		Default	
alt1.aspmx.l.google.com	5		Default	
alt2.aspmx.l.google.com	5		Default	
alt3.aspmx.l.google.com	10		Default	
alt4.aspmx.l.google.com	10		Default	
aspmx.l.google.com	20	mg	Default
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Hey, thanks mate! Ran into the same issue today with a comcast user and mailgun. Now I gotta figure out how best to approach MX records.

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Also wanted to express gratitude for sharing the answer here! It was an easy fix.

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