SMTP
ProtocolsSMTP is the standard internet protocol used to send email from a client or application to a mail server and between mail servers for delivery. It defines how messages are submitted, relayed, and queued, including addressing, routing, and basic error handling. SMTP typically works alongside other protocols for receiving mail and relies on authentication and encryption options to improve security.
How It Works
SMTP (Simple Mail Transfer Protocol) moves email using a store-and-forward model. A website, mail client, or application connects to an SMTP server, identifies itself, specifies the sender and recipient addresses, and transmits the message content. The server either delivers the message locally (if it hosts the recipient mailbox) or relays it to another SMTP server based on DNS MX records, retrying delivery when the destination is temporarily unavailable.
In hosting environments, SMTP is commonly used in two ways: message submission and server-to-server relay. Submission usually happens on ports 587 (or 465 for implicit TLS) and requires authentication to prevent abuse. Relay between mail servers typically uses port 25 and may be restricted by hosting networks to reduce spam. Encryption is added via STARTTLS or implicit TLS, and sender identity is validated with mechanisms such as SPF, DKIM, and DMARC to improve deliverability and reduce spoofing.
Why It Matters for Web Hosting
SMTP affects whether your site can reliably send transactional email (password resets, order confirmations, contact forms) and whether those messages land in inboxes. When comparing hosting plans, check if outbound SMTP is allowed, whether port 25 is blocked, what authentication and TLS options are supported, and whether you can use an external SMTP relay. Limits, logging, and queue visibility also matter for troubleshooting and deliverability.
Common Use Cases
- Sending transactional emails from web apps (WordPress, Laravel, Django)
- Contact form delivery with authenticated SMTP instead of PHP mail()
- Relaying email through a dedicated SMTP service for better deliverability
- Automated notifications from servers and monitoring tools
- Mail server to mail server delivery for custom domains
- Testing email flows in staging environments using SMTP credentials and logs
SMTP vs IMAP
SMTP is for sending and relaying email, while IMAP is for accessing and synchronizing mailboxes on a server. In practice, a typical setup uses SMTP to submit outgoing messages and IMAP to read incoming messages across devices. For hosting decisions, SMTP support determines outbound sending reliability, whereas IMAP support determines how users connect to hosted mailboxes and manage folders, flags, and message sync.