→ TCP is used in transport layer to carry out mail which is initiated by application layer protocol SMTP.
Primary protocols for E-Mail management
Post Office Protocol (POP)
Simple Mail Transfer Protocol (SMTP)
Internet Message Access Protocol (IMAP)
The above protocols are Application Layer Protocols
→ Once a client connects to the E-Mail Server, there may be 0(zero) or more SMTP transactions. If the client has no mail to send, then there are no SMTP transactions. Every e-mail message sent is an SMTP transfer.
→ SMTP is only used to send (push) messages to the server. POP and IMAP are used to receive messages as well as manage the mailbox contents(which includes tasks such as deleting, moving messages etc.).