SMTP, or Simple Mail Transfer Protocol, is the set of rules that email servers use to pass messages to each other. Every email you send, whether from Outlook, a website contact form or an online store, is handed from server to server using SMTP until it reaches the recipient’s mail provider. You rarely see it, but it’s the reason an email from a shop in Adelaide can land in an inbox in Brisbane, or Berlin, in seconds.
How SMTP works, step by step
- Your app connects. Your email program or website connects to an SMTP server and logs in.
- It introduces the message. It tells the server who the email is from and who it is for.
- It hands over the content. The subject, body and attachments are transferred.
- The server looks up the destination. It checks the recipient domain’s DNS records to find which server accepts its mail.
- The receiving server decides. It checks authentication and reputation, then accepts, rejects or files the message as spam.
SMTP only sends. Reading email uses different protocols, IMAP or POP3. See our FAQ on the difference between SMTP, IMAP and POP3.
Terms you’ll meet in settings screens
- Port: the “door” used to connect. 587 (with STARTTLS) and 465 (implicit TLS) are standard for authenticated sending. Port 25 is used between servers and is often blocked for home and small-office connections.
- Authentication: a username and password so only you can send through the server.
- TLS: encryption of the connection. Mailpro supports TLS 1.3.
- SPF, DKIM and DMARC: DNS records that prove your domain authorised the sending server. Our SPF, DKIM and DMARC guide explains them.
- Relay: a server that accepts your mail and delivers it onwards. See the definition of an SMTP relay.
Why businesses use an SMTP relay
Many Australian businesses send automated email through their web host or a staff mailbox. These are designed for person-to-person email, not for hundreds of invoices or order confirmations, and they often have low sending limits and shared reputations. When one customer on a shared host sends spam, everyone’s mail can suffer. A dedicated SMTP relay such as Mailpro’s SMTP server gives you authenticated, logged, trackable sending.
SMTP and Australian law
The protocol itself is neutral, but what you send through it isn’t. Under the Spam Act 2003, any commercial electronic message needs consent, clear sender identification and a working unsubscribe, whether it goes out through SMTP, an API or a newsletter tool. And because every message passes through your provider’s servers, the Privacy Act 1988 is relevant too: know where your provider stores data. Mailpro stores it in Switzerland on private infrastructure, outside US jurisdiction.
A quick example
A physiotherapy clinic in Canberra uses booking software that sends appointment confirmations. Previously they went out through the practice’s mailbox and often landed in spam. After switching the software’s SMTP settings to Mailpro and adding the SPF and DKIM records, the clinic can see in the log that each confirmation was delivered, and can check quickly when a patient says they didn’t receive one.
Next steps
If you want to see the data an SMTP relay provides, read about SMTP statistics. If you’re comparing sending methods, see API versus SMTP.