Which protocol is used by a server that needs to send an email message to another server in order to support successful delivery of the message?

The protocol used by a server to send an email message to another server to support successful delivery of the message is the Simple Mail Transfer Protocol (SMTP). Here are the steps involved:

1. The process begins when you compose an email on your email client and hit send.

2. Your email client communicates with the SMTP server and transfers the email to it.

3. The SMTP server verifies the outgoing email and recipient's email address. If the recipient's server is on a different domain, the SMTP server uses DNS (Domain Name System) to look up the recipient's SMTP server.

4. Once the recipient's SMTP server is found, your SMTP server forwards the email to that server.

5. The recipient's SMTP server then sends the email to the recipient's email client, which is then displayed in their inbox.

So, in summary, SMTP is the protocol that is used by servers to send email messages to each other to ensure successful delivery.