If I take any host from


why I'm not able to telnet to any of the specified ports ? I'm getting:

"Connecting To pop.1and1.com...Could not open connection to the host, on port 995 : Connect failed"

When I try to telnet on port 25 to SMTP Server mail.o2online.de port 25 I can see in Wireshark that the server responds with an RST/ACK packet so the port must be closed, so how can one send e-mails when it is closed?



My guess is that you are behind some firewall which is blocking the connections. I have no problems in accessing these hosts.

how can one send e-mails when it is closed?

If you are behind some company firewall you are probably supposed to use the companies internal mail server only which is usually not blocked.

  • This might be a very valid point. I'm puzzled though, because the reponse packet to the initial SYN has a TTL of 54 (based on the L2 address it's coming from a CISCO device) . . . so is it possible that it is that small or was it already decremented by on the way back ? – cyzczy Feb 17 '17 at 10:25
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    @adam86: unless the firewall is in your local network the L2 address you see is not from the firewall but from your local router. As for the TTL: hard to tell but in theory it can be set to anything by the firewall. – Steffen Ullrich Feb 17 '17 at 11:03
  • Thank you, it makes sens. My company is using Office 365 and I connect to it via Outlook 2013. I'm just wondering why would they block this ports, only to "force" me using the internal one ? I can still access gmail via the Web browser.... – cyzczy Feb 17 '17 at 11:18
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    @adam86: one of the reasons to restrict access to incoming mail servers is to decrease attack surface from phishing mails on private accounts. There probably scan web mail too. As for restricting outgoing mail: prevent data leakage and misuse by sending spam. – Steffen Ullrich Feb 17 '17 at 12:13
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    @adam86: correct, you should be able to create a TCP connection to it. – Steffen Ullrich Feb 17 '17 at 12:53

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