Which protocol is most commonly used by Secure Email Gateways for email transmission?

The most commonly used protocol by Secure Email Gateways for email transmission is Simple Mail Transfer Protocol (SMTP). SMTP is specifically designed for sending and routing emails between servers, making it the backbone of email communication on the internet. It facilitates the transfer of email messages from the sender's mail server to the recipient's mail server and is essential for sending outbound emails.

Secure Email Gateways utilize SMTP due to its compatibility with email clients and servers, allowing for the efficient handling of email traffic. Additionally, SMTP can be secured using various measures, such as TLS (Transport Layer Security), which ensures that the connection between mail servers is encrypted, thereby protecting sensitive information within the email content during transmission.

In contrast, FTP is primarily used for transferring files over the internet and is not suited for email communication. HTTP is utilized for web browsing and does not play a role in email transmission. POP is a protocol that retrieves emails from a server rather than sending them, therefore it does not apply to the transmission process managed by Secure Email Gateways. This context highlights why SMTP is the correct choice for email transmission in the framework of Secure Email Gateways.