What is the current version of the protocol that resides on an incoming mail server, similar to POP?

The correct response is IMAP4 because it is the current version of the Internet Message Access Protocol used for retrieving email from an incoming mail server. IMAP4 allows users to access and manage their email on the server rather than downloading it to a specific device, facilitating access from multiple devices and ensuring messages remain synchronized across them.

In contrast, SMTP (Simple Mail Transfer Protocol) is primarily used for sending outgoing mail rather than retrieving it, which distinguishes its function from both POP (Post Office Protocol) and IMAP. FTP (File Transfer Protocol) is designed for transferring files between computers over a network and has no relation to mail protocols. PPP (Point-to-Point Protocol) is used for direct connections between two nodes in a network, often in dial-up internet scenarios, making it irrelevant to email retrieval processes. These distinctions clarify why IMAP4 is the correct answer concerning its role as an incoming mail protocol.