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The standard port that is used to connect to the SMTP protocol is port 25. One of the most likely and common causes of errors when trying to send mail is blocking of port 25 by the Internet provider. In this situation there are two options. The first is to use a VPN, the second is to use an alternative port.
To check whether port 25 is blocked by your Internet provider, run the following command:
Windows
Start -> All programs -> Accessories -> Run -> cmd -> telnet your_site.com 25
In English:
Start -> All programs -> Basic -> Run -> cmd -> telnet your_site.com 25
MacOS
Mac HD -> “Applications” directory -> Utilities sub-directory telnet your_site.com 25
Linux
Open a terminal and enter the command:
telnet your_site.com 25
Test results:
If port 25 is open, you will see a similar message with the server response:
220 example.com ESMTP Postfix
If such a message does not appear, but you see a connection error message, the port is blocked!
Why is port 25 blocked? Most Internet providers block port 25 because it is used to send spam messages.
