The Simple Mail Transfer Protocol (SMTP) is the protocol used to send messages from one email server to another. Typically, these servers use the standard TCP port 25 designated for SMTP to transmit email. Many ISPs and cloud service providers however block outbound traffic on port 25, making it hard to test SMTP server implementations and deployments using regular networks.
SMTP Testing Tool allows you to test SMTP communication with your email server and make sure that the SMTP server is configured correctly. This is helpful if you are having trouble sending or receiving messages with your email server or if you are testing a custom messaging server implementation.