Which older email protocol operates over port 110 and only allows emails to be downloaded to one device at a time?

The correct answer is Post Office Protocol 3 (POP3). POP3 is specifically designed to download emails from a mail server to a local device, with the email that is downloaded being removed from the server in the default configuration. This means that the emails are primarily accessible on that single device where they are downloaded, effectively allowing users to work with their email offline.

Port 110 is the standard port used by POP3 for initiating connections and transferring messages. While it is possible to configure some settings to keep copies of emails on the server, the core functionality of POP3 revolves around the concept of one device accessing emails at a time, leading to a more simplified email management experience compared to protocols that allow for multiple simultaneous access, like IMAP.

The other protocols listed each serve different purposes. IMAP, for instance, allows multiple devices to access the same mailbox simultaneously, enabling users to manage emails across various devices effectively. SMTP is used for sending emails rather than receiving them, while Exchange ActiveSync is a protocol for synchronizing mailbox data, including emails, contacts, and calendar items, across devices, operating primarily in environments using Microsoft Exchange servers.