Testing to see if your ISP is blocking outgoing SMTP connections

Oftentimes to reduce the threat of spambot outbreaks coming from their network, ISP’s today will prohibit outgoing connections to port 25 except when connecting to the mailservers designated for their own clients to use. Some ISP’s will make exceptions if you contact them about it, but usually the person who answers their first tier support calls knows little about how SMTP works, or why you’d want to connect to some other SMTP server to begin with.

When sending email using a server on someone else’s network the errors in your own mail client may be less than helpful, saying ‘Server not available’ or something similar, leading the investigation in the wrong direction: towards the server you are connecting to rather than your ISP or local network.

Luckily many servers nowadays will have an alternate port open for this purpose (since a spambot is connecting to an MX server instead of your outgoing server they are much less likely to attempt connections to an alternate port as well). This is often set to port 587 though technically any port could be used.

Assuming SSL is not required, to test whether you are able to connect to a mailserver on port 25 simply open a command prompt and type ‘telnet someserver.yourhost.com 25′ (replacing the hostname with whatever outgoing server is provided by your host):

C:>telnet someserver.yourhost.com 25

220 someserver.yourhost.com (IMail 8.21 57484-4) NT-ESMTP Server X1

If the screen refreshes and you see an SMTP banner as above, then that port is open for you to connect to and the issue lies elsewhere.
If you know that this server is working for other people and the port is being blocked however, you’ll see a connection failure just as if the server is down or not responding:

C:>telnet someserver.yourhost.com 25
Connecting To someserver.yourhost.com…Could not open connection to the host, on port 25: Connect failed


Ask your email provider for an alternate port to connect to and any other relevant settings. Assuming SSL is not required you can test to this alternate port in the same way as above.

C:>telnet someserver.yourhost.com 587

220 someserver.yourhost.com (IMail 8.21 57809-2) NT-ESMTP Server X1

Note: The telnet program is not installed by default on recent Windows operating systems though it can easily be installed through Programs and Features by selecting ‘Turn Windows features on or off’ and checking ‘Telnet Client’, then Ok.

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