Final answer:
The protocol that syncs messages across multiple devices is IMAP protocol.
Step-by-step explanation:
The protocol that syncs messages across multiple devices is called the IMAP protocol. IMAP stands for Internet Message Access Protocol. It allows users to access their emails on different devices and keeps the messages synchronized across those devices.
For example, if you read an email on your phone, it will also appear as read on your computer. IMAP keeps track of the actions performed on the messages (such as read, flagged, or deleted) and updates the status across all devices.