Introduction

Telnet is a useful network tool that allows users to test connectivity with services on other servers. Using Telnet on port 25, it is possible to verify connectivity to an SMTP mail server, which is crucial for diagnosing email sending issues.

1. Create a Telnet Call on Port 25

To perform a basic test using Telnet on port 25, follow these steps:

Step 1: Open the terminal on your CentOS system.

Step 2: Type the following command:

telnet 25

2. Specify the Outgoing IP Address in the Telnet Call

If you need to specify an outgoing IP address for the Telnet connection, you can do so using the -b option. This method is useful when you have multiple IP addresses on your server and need to control which IP the connection is made from.

Step 1: Ensure that the IP address you want to use is properly configured on your server.

Step 2: Use the following command to initiate Telnet from a specific IP address:

telnet -b 25

Conclusion

Using Telnet to test SMTP connectivity can help you diagnose and resolve communication issues with mail servers. Remember that Telnet does not encrypt your connection, so any information you send or receive during the session could be intercepted by third parties. Always consider security when conducting these tests and use more secure tools when necessary.