You're setting up private email on a coworker's phone. He  wants to ensure that all of his mail is stored solely on their phone. Which mail protocol stores all emails on a single device and allows only one device to access them?

POP3 (Post Office Protocol 3) stores emails on a single device. This means that all email is accessed from that single device, which stores both sent and received messages. IMAP (Internet Message Access Protocol) downloads messages but stores a copy on the server. The SMTP (Simple Mail Transfer Protocol) protocol is used to send (but not receive) email. S/MIME (Secure Multipurpose Internet Mail Extensions) is an email encryption standard.