Which protocol does Darlene need to troubleshoot to send e-mail?

To send email messages, the correct protocol to troubleshoot is Simple Mail Transfer Protocol (SMTP). SMTP is specifically designed for the transmission of emails over the Internet. It operates by allowing the sending email server to communicate with the receiving email server, facilitating the process of sending messages from the user's email client to the destination server.

When troubleshooting email sending issues, you need to focus on SMTP because it handles the outbound traffic needed for delivering emails. This includes verifying the server settings, authentication processes, and ensuring that there are no blocks or errors in message routing.

While other protocols like Post Office Protocol (POP3) and Hypertext Transfer Protocol (HTTP) are important for receiving emails and web traffic respectively, they do not play a role in the actual sending of email messages. File Transfer Protocol (FTP) is utilized for transferring files between computers, which is unrelated to email functionality. Therefore, focusing on SMTP is critical in resolving issues related to sending emails.