Is your ISP blocking outgoing port 25?

Eric J Esslinger eesslinger at
Fri Jun 19 16:54:13 UTC 2009

I am the ISP, and we currently don't. However, I inherited this setup and have been slowly fixing glaring holes (those are fairly well gone now) and not so glaring one.  When our new firewall gets in, I will be rolling in port 25 blocks on dynamic IP addresses. The static ips will be unfiltered. Customers may send outbound mail through our SMTP server, or connect via alternate ports to their SMTP server.

From: Zhiyun Qian [zhiyunq at]
Sent: Thursday, June 18, 2009 2:36 PM
To: nanog at
Subject: Is your ISP blocking outgoing port 25?

It has been long heard that many ISPs block outgoing port 25 for the purpose
of reducing spam originated from their network.

I wonder which ISPs are still doing so. I know comcast has been doing that
but they cancelled it after many complaints. It seems to be the same case
for Verizon.

AT&T is the major one that I know of that is still enforcing this policy.
But they said they can unblock port 25 upon request. I am not sure how easy
it is.

One simple way to test if your ISP is blocking outgoing port 25 is to try:
"telnet 25" or "telnet 25". If
the connection fails, it could be due to the fact your ISP is blocking
outgoing port 25, although it can also be other reasons such as local
firewall configuration. Can someone perform the test and let me know result
if possible? Thanks a lot!


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