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I can telnet port 25 on mail.foo.com while I am at home. But I can't telnet to port 25 on mail.foo.com while I am on the internal network where the mail server is located.

This also applies for other ports like 443, which also means I can't access our webmail while on the internal network, but I can while on an external network as I have open port 443 on my firewall (dd-wrt).

I cal also ping mail.foo.com just fine.

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up vote 2 down vote accepted

You did not specify your topology so I'm guessing:

Hypothesis: You have your mail server NATed from the outside to its internal IP and you want to access it from the inside using it's outside IP.

The problem is that you NAT rule most probably is mapped to your outside network interface as well. This way it's not NATed and unless you have some egress filtering your connection goes outside to the net.

You either have to make a NAT rule for your "loop" connection and make all traffic cross you LAN twice or simply connect to it directly using its internal IP.

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