Which application layer protocol is primarily responsible for sending emails?

The Simple Mail Transfer Protocol (SMTP) is used for sending emails across networks. It operates at the application layer and is responsible for transferring email messages from a client to a server or between mail servers. SMTP is used to "push" email to a server, unlike protocols like POP3 or IMAP, which are used to retrieve emails from a server. Option A (HTTP) is incorrect because the HyperText Transfer Protocol (HTTP) is used for transmitting web pages, not email. Option B (FTP) is incorrect because FTP (File Transfer Protocol) is used for transferring files, not emails. Option C (DNS) is incorrect because DNS (Domain Name System) resolves domain names to IP addresses and is not involved in email transmission. Option E (POP3) is incorrect because Post Office Protocol 3 (POP3) is used to retrieve emails from a server, not to send them.