SMTP

SMTP (Simple Mail Transfer Protocol) is a protocol used for sending emails across the Internet. It operates on port 25 and is essential for email communication between mail servers. SMTP ensures that emails are properly routed and delivered to the recipient's mail server.

What is SMTP

SMTP, or Simple Mail Transfer Protocol, is a fundamental protocol used for sending emails across the Internet. It was first defined in 1982 by RFC 821 and has since become the standard for email transmission. SMTP operates on port 25 and is responsible for the reliable and efficient delivery of email messages from the sender's mail server to the recipient's mail server.

The protocol works by establishing a connection between the sending and receiving mail servers, allowing the transfer of email data. SMTP uses a series of commands and responses to facilitate this communication, ensuring that emails are correctly formatted and delivered. It is a text-based protocol, meaning that the commands and responses are human-readable.

SMTP is widely used by email clients and servers, including popular services like Gmail, Outlook, and Yahoo Mail. It plays a crucial role in the overall email infrastructure, enabling users to send messages to recipients anywhere in the world. Despite its importance, SMTP is often targeted by spammers and malicious actors, making it essential to implement security measures such as authentication and encryption to protect email communications.

SMTP runs on TCP and UDP port 25 and is official IANA port.

Security risk

If port 25 is open and the SMTP service is running, it can be exploited by spammers to send unsolicited emails, leading to potential blacklisting of the mail server. Additionally, without proper security measures, sensitive email data could be intercepted or tampered with by malicious actors.

In our UFW tutorial you can follow instructions how you can configure UFW to close port 25. Use StackChanges to monitor if the SMTP port is closed and stays closed. StackChanges will send an alert if port 25 is open again.