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I have just installed an smtp relay server that will only be used by apps behind our firewall to send email. I've called it and this is a valid dns entry that is pointing to my firewalls public ip.

It seems to work fine and I can send mail to gmail, hotmail and other domains.

My problem is that my ISP is blocking incoming port 25 traffic, so no one is able to connect from the net to on port 25. Could this be a problem? Do some servers connect back to verify that the sender server exists? Is there a risk that we could be marked as a source of spam?

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up vote 3 down vote accepted

Sites using 'sender verify' as an anti-spam measure will try to connect to the SMTP server(s) referred to by the sender's domain's MX records. As long as your outbound server isn't listed in the DNS as a mailserver, your setup is fine. Plenty of large organisations use separate outbound SMTP servers which aren't accessible from the outside.

There is a small risk that the IP address of your server may be listed as a spam source by one or more of the publicly available blacklists. Your ISP may expect its customers to send mail via its own SMTP servers, and might submit its customer netblocks to the blacklists as a pre-emptive measure to prevent spam originating from its network. You might find it easier simply to relay the mail through your ISP's customer-facing servers.

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Thanks for the reply. Our domains MX record points to another server that will accept port 25 connections. So I guess we will be fine. – 0tto Mar 1 '13 at 13:07

Yes, this could be a problem and although port 25 is blocked many use port 587 for the same thing (outgoing email).

Do some servers connect back to verify that the sender server exists?

You need to look into GreyListing and the "triplet" to understand more about how the server may communicate with other servers! Like all of these things, the configurations are bespoke (ie GreyListing may not be used).

From Wikipedia:

A server employing greylisting deliberately degrades mail service for unknown or suspect sources, over a short period of time. Typically, it records three pieces of data, known as a "triplet", for each incoming mail message:

The IP address of the connecting host
The envelope sender address
The envelope recipient address(es), or just the first of them.

So after the email is received, the server may wait 15 minutes before releasing the email to your mail box but before it does, it will check if the IP still exists, as does the sender and recipient.

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Thanks for the reply Dave, as soon as I have enough reputation I will mark your answer as useful. – 0tto Mar 1 '13 at 13:08

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