SMTP (Simple Mail Transfer Protocol)
SMTP is the standard protocol for sending email messages across the internet. Defined in RFC 5321, it handles the transmission of emails from the sender's mail client to the outgoing mail server and between mail servers until the message reaches the recipient's server.
Key Aspects
Internet Standard
The universal protocol used by virtually all email systems to send and relay messages across the internet.
Push Protocol
SMTP pushes email from sender to receiver; it does not retrieve messages (that is handled by IMAP or POP3).
Port Configuration
Commonly uses port 25 (server-to-server), port 587 (submission with authentication), and port 465 (implicit TLS).
How SMTP Sends Email
Client Submits Message
Your email client connects to the outgoing SMTP server (usually on port 587) and authenticates with your credentials.
Server Looks Up Destination
The sending SMTP server queries DNS for the recipient domain's MX (Mail Exchange) record to find the destination server.
Server-to-Server Transfer
The sending server connects to the recipient's mail server (on port 25) and transfers the message using SMTP commands.
Message Delivered to Mailbox
The recipient's mail server stores the message, which the recipient can then retrieve using IMAP or POP3.