SMTP, the Simple Mail Transfer Protocol, is the standard that email servers use to pass messages to one another. It dates back to the early 1980s and, in updated form, still carries virtually every email sent today, from a newsletter to a password reset. Our glossary has a short definition of SMTP; this page explains how it works in practice.
How a message travels
- Submission: your email program or application connects to an outgoing SMTP server and authenticates with a username and password.
- Lookup: the server checks the recipient’s domain (for example gmail.com) in the DNS to find which server accepts its mail.
- Transfer: the sending server connects to the receiving server and hands over the message.
- Checks: the receiving server verifies authentication records, reputation and content before deciding between inbox, spam folder or rejection.
- Retrieval: the recipient reads the email through IMAP, POP3 or a webmail interface. SMTP’s job ends at delivery.
Ports, and why port 25 may not work
Servers exchange mail with each other on port 25, but applications submitting mail normally use port 587 with STARTTLS encryption, or port 465 with implicit TLS. Many internet service providers and cloud hosts in the US block outbound port 25 from customer connections to limit spam from compromised machines. If your website or office equipment cannot send email, the port is a good first thing to check. Our article on port 465 vs port 587 explains the difference.
Authentication: why it matters more than ever
SMTP was designed without built-in proof of who sent a message, which is why spam and spoofing became possible. Three DNS-based standards now fill that gap:
- SPF lists the servers allowed to send for your domain.
- DKIM adds a cryptographic signature to each message.
- DMARC tells receiving servers what to do when checks fail and sends you reports.
Gmail and Yahoo have required authentication since 2024, and bulk senders also need DMARC. Our guide to SPF, DKIM and DMARC covers setup.
SMTP, IMAP and POP3: who does what
People often confuse the three. SMTP sends and transfers mail between servers. IMAP and POP3 are used by email programs to retrieve messages from a mailbox: IMAP keeps messages synchronized on the server across devices, while POP3 downloads them to one device. A marketing or transactional sending service only needs SMTP. Our FAQ on the difference between SMTP, IMAP and POP3 goes further.
SMTP server vs SMTP relay
Every email system has an SMTP server. An SMTP relay is a service that accepts mail from your applications and delivers it on your behalf, handling authentication, retries, bounce processing and reputation. For a small business that sends invoices, website notifications or order confirmations, a relay is usually more reliable than a shared hosting server. Mailpro’s relay uses deliverability routing and gives you delivery reports for every message.
SMTP and US law
The protocol itself is neutral, but what you send through it is not. Promotional emails sent via SMTP must meet the CAN-SPAM Act’s requirements, including an opt-out and your physical postal address, while transactional messages are largely exempt. Accurate header and routing information is required in both cases.
Getting started
To route your business email through an authenticated relay with data hosted in Switzerland, see the Mailpro SMTP server page, or compare it with the email API if you are building your own application.