Is your ISP blocking outgoing port 25?

Joe Provo nanog-post at
Thu Jun 18 20:14:45 UTC 2009

On Thu, Jun 18, 2009 at 03:36:44PM -0400, Zhiyun Qian wrote:
> It has been long heard that many ISPs block outgoing port 25 for the purpose
> of reducing spam originated from their network.

Yes, it is standard practice for non-server accounts and most dynamic-only 
accounts; only allow unauthenticated smtp traffic to your own smtp servers.  
If you are not running server-to-server traffic at the end of that broadband
pipe, then you should be shifting your userbase to authenticated on the SUBMIT 
port [587] anyway...
             RSUC / GweepNet / Spunk / FnB / Usenix / SAGE

More information about the NANOG mailing list