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I have 2 applications at the same IP with 2 different domain names. Hourjar uses noreply@hourjar.com for the "From" field when it sends emails, and Nimikri uses noreply@nimikri.com. If my my reverse DNS points back to nimikri.com, will my message from noreply@hourjar.com be marked as spam?

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As there is no one set of rules for distinguishing spam from ham the question can only be accurately answered in respect to specific anti-spam systems. –  John Gardeniers Jun 13 '10 at 6:11

3 Answers 3

Generally no, the Reverse DNS check is done on the sending mail server and what it sends it's name as part of the HELO (or EHLO). Now there is the possibility that they have checks that would count against you for not matching the From: field, but in reality these are set so low they might as we be turned off since it is very common for the domain name of the sending server to not match the domain in the From: field.

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returnpath.net do list based on RDNS.

"Data reported to our systems indicates that this particular host may be sending mail without permission from the service provider. Dynamic broadband connections sending mail can represent a compromised host sending spam via spambot, and are generally prohibited from mail delivery. As a result, the IP was added to the blacklist. If the IP in question is in fact static, its rDNS entry may need to be altered to reflect this."

So they may not insist it matches but if it is static and non-matching they will flag your host as a spam source.

All very non-RFC compliant, and I've told them so, but I suspect they don't care.

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Every well run shared hosting shop will have their servers with names like serverXX.hostingcompany.com with the PTR records matching. Blocking based on the whether the PTR record domain matches the email's From: domain would affect a massive amount of legitimate mail.

You'd be better off making sure that:

  1. The PTR record matches the FQDN of the server, see the output of hostname -f
  2. Your SPF record specifies that the server in question is allowed to send the mail.
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