Is your ISP blocking outgoing port 25?

Paul Stewart pstewart at
Thu Jun 18 19:38:20 UTC 2009

We still do it and never get any complaints - we don't filter static IP
customers but dynamic customers can either use our SMTP relays or
alternate ports....


-----Original Message-----
From: Zhiyun Qian [mailto:zhiyunq at] 
Sent: Thursday, June 18, 2009 3:37 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
of reducing spam originated from their network.
I wonder which ISPs are still doing so. I know comcast has been doing
but they cancelled it after many complaints. It seems to be the same
for Verizon.
AT&T is the major one that I know of that is still enforcing this
But they said they can unblock port 25 upon request. I am not sure how
it is.
One simple way to test if your ISP is blocking outgoing port 25 is to
"telnet 25" or "telnet 25".
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
if possible? Thanks a lot! 



"The information transmitted is intended only for the person or entity to which it is addressed and contains confidential and/or privileged material. If you received this in error, please contact the sender immediately and then destroy this transmission, including all attachments, without copying, distributing or disclosing same. Thank you."

More information about the NANOG mailing list