SMTP, the Simple Mail Transfer Protocol, is the standard that email servers use to pass messages to each other. Every email you send uses it, whether from Outlook on a laptop in Charlestown or from a booking system in a data centre abroad. Mailpro’s glossary entry for SMTP gives a short definition; this page explains how it works in practice.
The journey of one email
Imagine a guesthouse on Nevis sending a booking confirmation to a guest in Toronto.
- Submission. The booking software hands the email to an SMTP server, logging in with a username and password.
- Checks. The server looks up where the guest’s domain accepts mail, using a DNS record called MX.
- Transfer. It connects to the guest’s mail provider and passes the message over, usually encrypted.
- Verification. The receiving provider checks whether the guesthouse’s domain authorises that server (SPF), whether the message has a valid signature (DKIM) and what the domain’s policy says (DMARC).
- Delivery. If all is well, the email lands in the inbox. If not, it may be filtered to spam or rejected, and a bounce comes back.
All this usually takes seconds, even though the message crosses thousands of kilometres.
Ports and encryption
SMTP servers listen on specific ports. Port 587 with STARTTLS and port 465 with implicit TLS are the usual choices for sending from software; port 25 is mostly for server-to-server traffic and is often blocked by internet providers. Mailpro supports encrypted connections with TLS 1.3. The trade-offs between ports are covered in port 465 versus 587.
Why a relay helps small island businesses
You can run your own SMTP server, but keeping it trusted by Gmail and Outlook requires constant attention to IP reputation and security. Most businesses in the Federation, where IT teams are small or outsourced, are better served by an SMTP relay: a service that sends on your behalf from well-maintained infrastructure. Mailpro’s relay also records delivery routing and results for each message.
SMTP, IMAP and POP3: what is the difference?
SMTP only sends. To read mail, your email app uses IMAP or POP3 to fetch messages from your mailbox. That is why an email client on a laptop in Basseterre has separate incoming and outgoing server settings. A relay like Mailpro replaces the outgoing side for bulk and system emails; your normal mailbox stays as it is.
Common problems and what they mean
- Authentication failed: the username or password in your software is wrong, or the port and encryption settings don’t match.
- Connection timed out: the internet provider or firewall is blocking the port; try 587 or 465.
- Message rejected: the receiving server did not trust the message, often because SPF or DKIM is missing for your domain.
SMTP and your data
Every email carries personal data: names, addresses, sometimes booking details. The Federation’s Data Protection Act, 2018 asks businesses to protect personal data, including by considering where it is stored and making sure processors give proper security guarantees. Mailpro processes and stores data in Switzerland, outside US jurisdiction.
To start sending through a relay, see Mailpro’s SMTP server for St Kitts and Nevis. Developers comparing options may also want API versus SMTP. Volume-based plans are on the pricing page.