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Assume I own the domain name, example.org. I am creating a setup where I receive email sent to me@example.org using Google Apps. The MX records for example.org already point to Google's servers and it works great. Now, I have an application server where I would like to send out automated emails, such as notifications about user activity, etc. I may also use this server to send bulk emails such as newsletters (legitimately, of course). Here, I have the option to use a subdomain for all of my automated outgoing email. Perhaps I can send these emails from service@mail.example.org and set the Reply-To header to service@example.org, which would then be directed to my Google account for a real person to answer.

My question is: Is there any added benefit of sending automated email from an email address with a subdomain?

Note: I have already taken measures to use SPF and DKIM to authenticate my email from the application server. I have heard that some receiving mail servers also do a reverse MX check to determine whether your email is spam. If this is true(?), I'm assuming a subdomain would be useful because I can then make a separate MX record for mail.example.org to point to my application server, while keeping the other MX's for example.org on Google.

Aside from that, is there any other reason I would use a subdomain? I notice this is commonly used in email from Groupon, Southwest Airlines, Office depot, and countless other newsletter services (sometimes they use entirely different domains), and I haven't been able to figure out why.

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You do not need a MX record to send mail. –  gparent Sep 7 '13 at 0:53
I understand that an MX record is not necessary to send mail, but should I worry about receiving mail servers performing a reverse MX check and finding the IP of the MX to be different from where the email was sent from? –  Michael Brook Sep 7 '13 at 2:28
You don't need to write your SPF record against the MX record. You can also use IP addresses. –  gparent Sep 7 '13 at 16:45

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