mail server DNS configuration questions

Andrew Falanga af300wsm at
Thu Sep 11 01:33:40 UTC 2008

Sahil Tandon wrote:
> Andrew Falanga <af300wsm at> wrote:
>> It doesn't take a rocket scientist, or a computer scientist, to 
>> figure out we've got DNS issues.
> What exactly is the problem though?  What problems are you having on 
> the mail server that lead you to the above conclusion?
Clients in the churches private network cannot send mail using this 
server, though they can receive mail from it (POP).  The church has a 
private network, PN1, and the mail server sits at a church members house 
because he has a static IP address; let's call that PN2.  The router at 
his house is setup to forward traffic over port 25, and the POP port, to 
this server.  Also, just to further clarify, the Internet separates 
these two Private Networks.  However, this may not be entirely true as I 
think about it because at both locations, the ISP is CableOne using 
cable broadband.  So, though technically part of the Internet, the 
traffic shouldn't leave the CableOne domain.  Also, of interest, is that 
another of our pastors uses CableOne at home and is unable to send 
e-mail using the churches server from home.  However, from a coffee shop 
in town, that our pastors frequent, they are able to send mail.  It is 
my understanding that this coffee shop does not use CableOne.

So, just to make sure everyone's got it, the mail server sits in PN2.  
While diagnosing this, I connect to the server (using Putty) from a 
machine in PN1, using either a mail client or telnet I'm unable to make 
a connection to the mail server over port 25.  Using tcpdump during this 
putty session I do not even see the SYN packets for the start of the 
connection from the machines in PN1.  This is only when connecting to 
port 25.  Obviously, I can connect to the server because I'm using 
putty.  Also, I can see the SYN packets for the start of the connection 
when this same machine in PN1 attempts to connect to port 80.  The 
problem seems to be when trying to connect over port 25.  For some 
reason, the packets aren't being delivered to that address 
(  This happens if I try to telnet to or telnet to on port 25.  The 
packets aren't being delivered.  They're being sent somewhere else, or 
lost in digital purgatory.

Now, from home (my home) let's call this PN3, I can send/receive mail 
using the church e-mail server.  I, however, don't use CableOne.  Are 
there routers that route traffic based on port number?  It's almost as 
if traffic, that originates within the CableOne domain and travels 
through, but not outside, the CableOne domain, doesn't get routed to the 
correct address when it's destined for port 25.


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