Is your ISP blocking outgoing port 25?

Charles Wyble charles at
Thu Jun 18 19:55:09 UTC 2009

Do you provide your users an SMTP server to use, with some out bound 
spam filtering?

It would seem this is to be expected, as you don't want your IP ranges 
showing up on RBL filters.

Do you force SSL connectivity like AT&T does?

Paul Stewart wrote:
> 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....
> Paul
> -----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
> 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! 
> Regards.
> -Zhiyun
> ----------------------------------------------------------------------------
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