Email client programs like Outlook, Pegasus, Eudora, and Thunderbird send emails through the server using TCP port 25. However, this port is often used by malware. In order to protect the network, office mail servers or mail servers that are hosted for a domain may block port 25. To send emails in situations like this, TCP port 587 must be open. To open port 587, you have to change your firewall’s settings so it knows port 587 is an exception and doesn’t block it.
Click “Start,” “Control Panel,” “Security,” and “Windows Firewall” to get to the Windows Firewall interface.
Click “Allow a program through Windows Firewall.” You can find this option in the top left corner of the window.
Click on “Add port,” displayed in the bottom region of the Windows Firewall Settings pop-up box.
In the “Name” field, type the name you want for port 587. In the “Port number” field, type “587,” and for “Protocol,” click “TCP.” Port 587 is a TCP protocol port because it establishes a connection between two hosts so data streams can be exchanged. Click “OK” to let your network use port 587.