Windows 7 is blocking telnet 25

Is the server you are trying to connect to inside your network?

Since you mention port 25, my guess is that it isn't and you are trying to access some external mail server. Most ISPs these days block outgoing port 25 as an anti-spam strategy. You could try contacting your ISP to have the filter removed, but mostly they will only do that if you are paying for a business class connection.

As for how to diagnose, if this isn't internal, my suggestion would be to setup an internal computer to listen on port 25. If you can connect to that, that would confirm that it isn't your OS firewall that is blocking anything. You could also rule out your OS firewall by booting up a Linux livecd or something and testing from that.

Zoredache

Posted 2017-02-24T22:02:29.663

Reputation: 18 453

Than how a mail client is able to connect to a SMTP server if I can't telnet to it? – sebastian_t – 2017-02-24T23:03:27.073

1Are you sure your mail client is able to connect from within your network, and that it is using port 25? Anything using exchange would be going over http(s), receiving mail is either through pop/imap. Only stuff you send, that is not going through the alternate submission port would use port 25. – Zoredache – 2017-02-24T23:06:18.580

The use of port 25 is commonly restricted to reduce the ability of botnets and other spammers. Are you trying to connect to your work SMTP server on port 25, if so you may also be battling against the company firewalls (they're letting office PCs in but not internet PCs). Are you trying to get to a well-known public SMTP server, if so it is most likely what Zoredache said and your ISP is preventing outbound connection on 25 as part of their network-protection-safety; a call to your ISP's tech support would answer that (ATT does this, for example) – Ruscal – 2017-02-24T23:59:30.457

Windows 7 is blocking telnet 25

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