Which protocol specifies how emails should be delivered from one mail domain to another and operates over port 25?

The Simple Mail Transfer Protocol (SMTP) is the protocol responsible for the transmission of email messages between servers. It operates primarily on port 25, which is the designated port for email delivery between mail transfer agents (MTAs). When an email is sent from one domain to another, SMTP governs the process of how that email is relayed and delivered to its destination.

SMTP works as a push protocol, meaning that it is used to send messages to recipients' mail servers. It establishes a connection between the sending server and the receiving server, ensuring that emails are transmitted accurately and in a sequence. This is essential for the overall functionality of email communications on the internet, making SMTP a fundamental component of email delivery systems.

In contrast, other protocols listed serve different roles in the email ecosystem. For instance, IMAP and POP are protocols used for retrieving emails from a mail server, not for sending them. IMAP allows users to access and manage their emails on a remote mail server, while POP is used to download emails for local storage. HTTP is a protocol for transfer of web pages and does not relate to email transmission.