Email client programs such as Outlook, Pegasus, Eudora and Thunderbird use TCP port 25 to send emails through the server; however, this port tends to get exploited with malware. Consequently, office mail servers or hosted mail servers for a domain may block port 25 to protect the network. In scenarios like this, TCP port 587 must be enabled to send emails. Enabling port 587 requires you to configure the settings of your firewall so it recognizes port 587 as an exception and doesn't block it.
To access your Windows Firewall interface, click "Start," "Control Panel," "Security" and "Windows Firewall."
Click on "Allow a program through Windows Firewall." This option is found in the upper left region of the window.
Click on "Add port," displayed in the bottom region of the Windows Firewall Settings pop-up box.
Enter the name you want for port 587 in the "Name" field. Enter "587" in the "Port number" field, and click "TCP" for "Protocol." Port 587 is a TCP protocol port because it establishes a connection between two hosts so data streams can be exchanged. Click "OK" to enable port 587 in your network.