How Email Flows Through The Internet & Blacklisting

By: Chris Lewis

Sometimes it's best to go back to basics to understand some of the issues that can hamper your email distribution efforts. One of those basics is just how email is sent through the internet to the destination.  You may set up the perfect email and it may never reach your destination because it was blocked from getting there.  Most of the "hoops" that need to be jumped through is because of SPAM and open SMTP relays (servers that will allow anyone to send from it).  Below is a quick explanation of how the email system works and how your email interacts with sending and receiving SMTP servers. Here is the path an email takes:

1. Your email client sends an email to your SMTP server, say it was sent to chris@theirdomain.com

2. Your SMTP server grabs the domain name in the email, in this case THEIRDOMAIN.COM

3. Your SMTP server looks up the MX records for THEREDOMAIN.COM from your DNS Server (this is the MXLookup you find in MXToolbox.com website)

4. In the order of priority, your SMTP server attempts to connect to the first destination SMTP server based on the first MX record.

5. If the first destination SMTP is unavailable or busy, your SMTP server will move on to the next destination SMTP server, and so on.

6. Once your SMTP server has connected to a destination SMTP server, before the email is transferred, the full return-path (or FROM) email address to is sent to the destination SMTP server

7. If the destination SMTP is setup correctly, it will take the return-path email address and extract the domain name (in our case ARIALSOFTWARE.COM if we were sending the email)

8. Now the destination SMTP starts authenticating the email by one or more methods. The most common are below:
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• The destination SMTP server looks at all it's subscribing blacklists and sees if your SMTP server's IP address is on the  list.  The destination SMTP server will most of the time reject any emails from any domain from your SMTP server
• The destination SMTP server "sees" the IP address of your SMTP server, which is does a reverse SMTP lookup to see  what domain name is assigned, and if the domain it finds does not match the domain of the return-path of your email than it can be rejected
• The destination SMTP server looks up the SPF record for ARIALSOFTWARE.COM from the ARIALSOFTWARE.COM  DNS entry.  If there is no SPF record for the ARIALSOFTWARE.COM domain, then your SMTP server may not have "authority" to send ARIALSOFTWARE.COM emails so the destination SMTP server can just reject it.  If the SPF record is found for ARIALSOFTWARE.COM, the record is examined to see if your SMTP server has authority to send  ARIALSOFTWARE.COM emails.  If your SMTP Server is not in the list, your emails can be rejected.
• A destination SMTP could have a built in SPAM scoring system like Assassin.  Your mail will be scored and either allowed to go through unscathed, allowed to go through but marked as SPAM, or not allowed to go through at all.

9. Any combination of these above methods plus other methods can be used to determine if an email is just rejected by the destination SMTP server
10. Exceptions - Sometimes these methods will create a SPAM score, and if the score is exceeded then the email is rejected.  So it is possible for one of these to fail but the email still makes it through
 
So, the answers to why an email does not make it is within this flow.  The MXToolbox.com website is a big combination of different tests which may confuse the issues.  Blacklisting ONLY has to do with the "sending" SMTP server and has nothing to do with a particular domain because you could use the same SMTP server to send out emails for multiple domains…so the IP address is king.  If you try to send out emails through your SMTP server that is blacklisted, most of the time the emails will be rejected by the destination SMTP server.

So, to effectively use the MXToolbox.com tool for blacklisting detection, you need to do two things.  First, you need to get the SMTP server they are using in Campaign Enterprise or Email Marketing Director.  You need to translate that SMTP server setting to an IP address if it is not already one.  Then you go to the Blacklisting tab and put that IP address in and then see if your SMTP server is blacklisted.

One last spin on this…if the SMTP server setting they are using is an "internal" IP address, like 10.10.10.1 or 192.168.1.100, then you will have to find out what the "external" IP address of the SMTP server is to see if it is truly blacklisted in the world.