SMTP

SMTP (Simple Mail Transfer Protocol) is the standard protocol responsible for sending and relaying email messages between mail servers. It transports a message from the sender through successive servers to the recipient's server — while reading the message from the mailbox is handled by other protocols, IMAP or POP3. SMTP is therefore the internet's "postmaster", making sure the item is delivered to the right address.

How SMTP works

Sending a message happens in several steps that involve the domain's DNS records:

1. a mail client or application connects to the SMTP server (usually on port 587) and hands over the message,
2. the sender's server checks the recipient domain's MX record in DNS to find the correct destination server,
3. the message is relayed to the recipient's server, possibly through intermediate servers,
4. the destination server accepts the item and places it in the recipient's mailbox.

Along the way, sender authentication mechanisms are verified: SPF, DKIM and DMARC, which guard against spoofing and influence whether the message reaches the inbox or the spam folder.

SMTP in practice

Every email send relies on SMTP: from an email client, through application scripts (forms, notifications), to newsletter systems. Configuration involves the SMTP server address, the port (587 with STARTTLS or 465 with SSL/TLS) and authentication credentials. For good deliverability, correctly configuring the domain's SPF, DKIM and DMARC records is essential — without them messages more often land in spam or are rejected. In hosting, an SMTP server is usually provided alongside mail accounts.