Explanation
SMTP stands for Simple Mail Transfer Protocol, which is used to relay email from a user's mail server to another mail server or from a mail client to a user's mail server. IMAP stands for Internet Message Access Protocol, which is used to access and manage email messages on a mail server. FTP stands for File Transfer Protocol, which is used to transfer files between computers over a network. POP3 stands for Post Office Protocol version 3, which is used to download email messages from a mail server to a mail client. References:
The Official CompTIA IT Fundamentals (ITF+) Student Guide (Exam FC0-U61), Chapter 5: Infrastructure Concepts1