SMTP, the Simple Mail Transfer Protocol, is the standard set of rules that email servers use to send messages to one another. Every email you send uses it, whether it is a booking confirmation from a Christ Church guesthouse or a statement from a Bridgetown financial firm. Our SMTP definition gives the short version. Here is how it works in practice.
The journey of one email
Imagine a villa’s booking system sends a confirmation to a guest in Manchester:
- Submission. The booking system connects to an outgoing SMTP server, logs in with a username and password, and hands over the message.
- Lookup. The SMTP server checks the DNS records of the guest’s email domain to find which server accepts mail for it.
- Transfer. The server connects to the recipient’s mail server over an encrypted link and transfers the message using SMTP commands.
- Checks. The receiving server checks the sender’s authentication (SPF, DKIM, DMARC), reputation and content, then decides: inbox, spam folder or rejection.
- Retrieval. The guest opens the email on their phone, which fetches it using a different protocol, IMAP or POP3.
The whole trip usually takes seconds, across the Atlantic and back.
Ports and encryption
SMTP uses network ports. Port 587 with STARTTLS and port 465 with implicit TLS are the usual choices for authenticated sending today. Port 25 is mostly used between servers and is often blocked for ordinary users. Our article on port 465 vs 587 explains the difference. Mailpro connections use TLS 1.3.
What an SMTP relay adds
You could run your own mail server, but keeping it off blacklists and correctly authenticated is a job in itself. An SMTP relay service such as Mailpro’s takes care of that. It also provides tracking, bounce handling and reporting. For small IT teams in Barbados, a relay is usually simpler and more reliable than maintaining a server in-house.
Why authentication matters more than ever
Major inbox providers now reject or filter unauthenticated bulk email. If your domain ends in .bb or .com and you send from several systems, such as a website, a booking engine and an accounting package, each needs to be covered by your SPF and DKIM records, with a DMARC policy on top. Our guide to SPF, DKIM and DMARC walks through the setup.
Common SMTP problems and what they mean
| Symptom | Likely cause |
|---|---|
| “Authentication failed” | Wrong username or password, or the wrong port and encryption combination |
| Connection times out | Port blocked by the network or internet provider; try 587 or 465 |
| Mail accepted but lands in spam | Missing SPF, DKIM or DMARC, or content that looks like spam |
| Bounce: “user unknown” | The recipient address is wrong or no longer exists |
The FAQ on common SMTP errors lists more codes and fixes.
SMTP, personal data and hosting
Every email carries personal data: at least the recipient’s address, and often names, booking details or amounts. Under the Data Protection Act 2019, you are responsible for how that data is handled, including by the service that relays it. You must also make sure any overseas destination offers adequate protection. Mailpro relays and stores data in Switzerland, outside US jurisdiction.
Next steps
See what Mailpro’s relay offers on the SMTP server in Barbados page, or learn which reports to watch in SMTP statistics.