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550 5.1.0 Invalid sender domain

This may be a known error but I found out thru the blogoshere that it looks like a Comcast problem. Is there a way to circumvent this issue? All my "comcast.net" emails are bouncing.

Here's what I found out:

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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