SMTP
SMTP is the standard protocol that sends and relays email between mail clients and mail servers across the internet.
What is SMTP?
SMTP (Simple Mail Transfer Protocol) is the internet’s standard protocol for sending and relaying email — both from a mail client to a mail server and onward between mail servers. SMTP only handles sending; retrieving received mail is done by other protocols such as IMAP and POP3. Authenticated submission from a client typically runs over port 587 with STARTTLS (or 465 with implicit TLS), while port 25 is mainly used for server-to-server relay.
Why does SMTP matter?
For an online store, email is business-critical: order confirmations, shipping notifications, and password resets have to reach the inbox every time. SMTP is the transport layer that delivers those messages, and setup mistakes show up as mail landing in spam or not being delivered at all. Because core SMTP does not verify the sender on its own, good deliverability depends on pairing it with DNS-based standards like SPF, DKIM, and DMARC.
Common use cases
- Sending transactional emails such as order confirmations and shipping notifications.
- Setting up a dedicated sending address in your email marketing software or ESP.
- Debugging mail that lands in spam or bounces due to missing authentication.
- Configuring port and TLS when integrating with an external mail service.
Shopify perspective
Shopify sends its own notification and transactional emails through its own infrastructure, so you generally cannot point Shopify at your own SMTP server. Instead, you authenticate your sender domain with SPF and DKIM records so mail sent from your store looks legitimate. If you use apps like Klaviyo or Omnisend, they send through their own delivery infrastructure — you only need to verify the domain, not run an SMTP server.