Ask the Network Guy!

I have had a similar experience with my DynDNS and email not flowing correctly.  In my case I was using home cable modem with a dynamic IP Address (usually why one uses a mailhop) and the cable modem began to block all outbound emails from my router.  The ISP took the MAC address of my router and blocked all port 25 traffic.  Allow me a moment to share some particulars about each of the following items:

• MAC Addresses
• Port 25
• Email flow

All NIC (Network Interface Controller), Routers, Access Points, Any other device that has an IP Address has a MAC address.  The Mac Address is a unique code to a particular item.  So there are no 2 MAC addresses the same.  Here is an example of a MAC Address: 

00-15-C5-3E-31-97.  The first 3 sets of numbers and characters are unique to a particular manufacture.  The last 3 sets are unique to each Network Interface Controller.

Email is transmitted via SMTP (Simple Mail Transport Protocol) over PORT 25 .  For example, when you go to the settings of your email software and setup the SMTP server settings for your POP3 email, the program knows to talk to that server on Port 25.

As we have seen port 25 is needed for email to flow, but what is really going on?  I will use a simple example; you open up Microsoft Outlook and the program then begins to download emails.  Well this all begins with the settings that you have entered into your software.  The program first looks at the SMTP server listed.  For sake of this example, your SMTP email server is smtp.comcast.net.  Outlook then begins to connect to the SMTP server on port 25, the email server then asks for your username and password.  Once both are authenticated, the emails are then downloaded into your email program.

So one can see how important this PORT 25 is to the flow of email.  DynDNS has a service that allows an email server to send email via Port 25.  The importance of this service is two fold.

1. To help reduce SPAM.  By using a DynDns service.  Emails have to originate on the local network before being allowed to be sent out.  Once okayed from the email server the server then sends the email to the outbound mailhop server, which also requires authentication.
2. Other email servers want to validate the email that you are trying to send.  You can send email without a DynDNS service and with a Dynamic IP address but there are some issues.  First, because you are on a dynamic IP address some email servers will not be able to authenticate that the email that you are attempting to send is valid.  So they will not allow this email to go through thus producing a Non-Deliverable report.  Second if you open up your firewall to allow outside email servers to authenticate to your server (allowing inbound Port 25 traffic).  Then you are allowing others to use your server to send out SPAM emails, and this will defeat the purpose.

The ISP blocked all outbound transmissions via the MAC address, and because as we learned above that they are all unique, even if the router obtains a new IP address, traffic would still be blocked.  So when the TCP/IP Port 25 is blocked from your local ISP on your Dynamic IP address, you can change the outbound port.  In my case I was able to send the emails to DynDns via port 2525.  Because this is not a formal email port, it was not blocked, the emails were able to be delivered to the mailhop on DynDns and the email queue was quickly cleared.