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I am concerned my mail server setup may cause some ISPs to reject mail that is sent by it.

I am using Postfix to send mail for a web application, light load, user notices. It also sends cron notices and "monit" alerts. It is for sending mail only. I guess it wants to be "null client".

The postfix server and webapp live on the same box. The webapp builds the "to" and "from" email mail headers.

Server: 123.foobar.com Website Domain: mysupersite.com

Mail that is getting sent by postfix has: from: foo@mysupersite.com HELO: 123.foobar.com (123.123.123.123) (I am not sure this is enough header information to describe my condition)

I have read that some mail servers will reject incoming mail where the "from" email address domain does not match the sending servers domain...

To throw a twist into the mix, this server does not have a dns record. I have read that some servers are configured this way on purpose.

For example: "dig 123.foobar.com +short" & "dig -x 123.123.123.123 +short" give no answer for my server domain.

So there seems to be no way for a mail server to see if HELO is telling the truth. IF the accepting mail server is even checking.

As of now, mail is getting sent as I describe and it arrives to its destination but only a small amount of test mails.

But is this the right way? Is there a better way? Should I send my mail through a relay host? I have more questions than answers. Please help!

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serverfault.com/q/48428/984 –  Zoredache Aug 19 '11 at 4:12

2 Answers 2

Typically, this type of email (server) will relay through a properly running SMTP server to reduce the chances of the emails getting filtered - something like your ISP's relay will usually work well enough.

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Looking more at my resources using a relay is not a good option right now. The example domain above, 123.foobar.com, has been given a dns record. So the mail server box can be resolved. Hopefully that will less the chance of getting filterd. –  Bullwinkle500 Aug 19 '11 at 18:13

There is absolutely no need to have matching server and email addresses. But:

If you want to send mails as someone@example.com and use a sending host named amailer.smtprelay.invalid there is nothing uncommon with that. This is how Millions of mails get send every day. Just look at Google Apps Mail, where the sending host is something ending with google.com whereas the hosted domain could be example.net.

The only requirements that you have to follow are that you have consistent DNS entries for example.com and amailer.smtprelay.invalid. That means that you have either a MX record or at least an A record for example.com (doesn't matter where they point to, but must be valid). And on top of that you need consistency in the DNS for amailer.smtprelay.invalid so that this name points to 0.1.2.3 and 0.1.2.3 has an PTR record to amailer.smtprelay.invalid. And the HELO/EHLO string of the server must also match with the name amailer.smtprelay.invalid.

There is one exception to that. If your sending server is not something like amailer.smtprelay.invalid but 3-2-1-0.dynamic.isp.invalid then you are lost as this is mostly an indication that your host is in a dynamic IP range so that many SMTP servers will refuse to accept your mails. Direct sending from dynamic IPs is not prohibited but not recommended.

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