Problems sending emails to external domains

If you face problems sending out emails, but only to a handful of domains, please run through the following checks / tests and make sure your environment is setup properly:

  • Check your domain on http://www.dnsstuff.com (subscription required) or http://www.mxtoolbox.com/diagnostic.aspx (free) and see if you have a Reverse DNS pointer setup.  If you do not have one setup – call your Internet Service Provider (ISP) and ask them to set one up to match the Fully Qualified Domain Name (FQDN) that your mail server responds as e.g., mail.yourcompany.com. Also, your mailserver FQDN should also be setup with something like mail.yourcompany.com.  Any FQDN ending in .local or .internal or anything that is not a valid Internet Domain Name is not correct and should be changed otherwise you may experience problems sending out emails to some domains.
  • Check that your IP address is not listed on any Blacklists on http://www.mxtoolbox.com/blacklists.aspx – If you appear on any blacklists, then you may have problems sending mail to some domains who check against blacklists (not everyone does, but a lot do).  Follow the links on the results page to the particular blacklist site to find out the reason why you are listed (you may have an infected computer sending out spam that you are not aware of) and then deal with the issue before requesting removal from those blacklists (if you don’t deal with the problem, such as an infected computer, you will get removed from the blacklist, but will only re-appear again as more spam is sent out).  Once you know what you are facing, you can resolve the problem.

If you are blacklisted – configure your firewall / router to block all traffic on TCP Port 25 Outbound from all IP addresses apart from your Mail Server.  This should reduce the possibility of an infection from getting you blacklisted further and will help prevent getting listed again once you have cleaned up your network.

  • Check your IP reputation on Senderbase http://www.senderbase.org/senderbase_queries/rep_lookup.  You will either be Good, Neutral or Poor.  If your reputation is Poor – then you may have problems sending out mail and are most likely appearing on a blacklist or two somewhere.  If you are Neutral, then you may have had a problem in the recent past and are still recovering your reputation.  If you have a Good reputation, you should have no problems sending out emails.
  • Check to see if you have an SPF (Sender Policy Framework) record setup on http://www.mxtoolbox.com/spf.aspx – If you do not have a record setup, visit http://old.openspf.org/wizard.html, run through the various options carefully and then you should see your SPF record in the final box at the bottom of the screen. Once you have an SPF record, you have to publish this record in your Domains DNS records by adding a TXT record with the SPF record as the data e.g., Type=TXT Record=(output from http://old.openspf.org/wizard.html). An alternative site to the openspf.org site that you can use is http://www.microsoft.com/mscorp/safety/content/technologies/senderid/wizard/
  • Check to make sure that the advertised IP Address in DNS for your primary MX record is the same IP address that you are sending mail from. Ideally – they should be the same for optimal mail-flow although if you are using a 3rd party spam filtering service or have inbound mail on one IP Address and outbound mail on another, this is not going to be possible.

If you do send out mail from a different IP address to the advertised MX record IP Address, please check that the Reverse DNS entry for this IP Address is also configured properly and that it resolves correctly to the same IP address (I use http://www.dnsstuff.com to check this – but you will need a subscription!). As an example, if you send mail out via IP 123.123.123.123 and the Reverse DNS entry setup on this IP address by your ISP is mail.yourcompany.com, mail.yourcompany.com should also resolve in DNS back to the same 123.123.123.123 IP Address.

Having checked all of the above and made any corrections to your configuration, your mail should be flowing better. If it is not and your house is now in order, you are not listed on any blacklists and you still have problems sending out mail to one or more domains, then it may be that the external domain may be specifically blocking you, (Hotmail are quite good at doing this for no particularly good reason) you will need to contact them to try to resolve the issue.

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