mail server DNS configuration questions

Kevin Kinsey kdk at
Thu Sep 11 02:44:09 UTC 2008

Andrew Falanga wrote:

> 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.

So a common thread is that traffic on the ISP's net isn't going
out via --- would seem to indicate port blocking,
which is quite common for port 25.  Tried 587 or some weird alternate?

Kevin Kinsey

If the odds are a million to one against something
occurring, chances are 50-50 it will.

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