I have 3 computer all running ubuntu, networked behind a linksys router that is connected to my ISP. I use one of the computers for development and to monitor - using nagios and postfix -the services running on the other two. The email was set to relay the emails through my ISP's SMTP server. Everything has been running ok until last week when I stopped receiving nagios emails.

I've discovered that I can no longer connect to my ISP's SMTP server and in fact any SMTP server (GMail, Yahoo, etc.) from the machine running nagios/postfix. I test the connection with: telnet smtp.gmail.com 587 and immediately get the "connection refused" message - no time spent "Trying..." at all. I've checked my external ip against spam blocklists but it's clean. The kicker is my telnet test works ok from any of the other 2 computers. I've tried changing the internal ip of the nagios/postfix computer but that didn't seem to help.

My questions are: Am I being blocked by my ISP? and if so, how can they block a specific computer behind the router? What identifies it externally as different from the other two? Hostname? MAC address?

Thanks for any help...

It sounds like you are being blocked at the ISP level.

I agree but how does know to block that one computer and not the others? I've changed the internal ip on the blocked computer and it was still blocked. I've also changed the hostname and it was still blocked. The only other thing I can think of is it's being blocked by it's mac address.

Can anyone verify?

Did you play with any firewall products on the postfix machine -- iptables, guarddog, shorewall, etc? If so, the rules may still be active and you're blocking yourself. ;-)

Could possibly have too many connections made. Have you tried looking in /var/log/messages or /var/log/mail. That is where I would start. Also try stopping and restarting the postfix daemon. I think you can do something like postfix reload or restart and that should do it for you. Normally people block by IP address not Mac address.