SMTP stands for Simple Mail Transfer Protocol. It is the set of rules mail servers use to pass an email from sender to recipient, and it has been doing that job since the early days of the internet. If your business sends email from a website, an online shop, a booking system or accounting software, SMTP is almost certainly involved. Understanding the basics helps you fix delivery problems and choose the right setup. For a formal definition, see our glossary entry on SMTP.
How an email travels
Imagine a customer books a kayak tour in Abel Tasman National Park through the operator’s website:
- The booking software creates a confirmation email and connects to an outgoing SMTP server with a username and password.
- The SMTP server checks the credentials, accepts the message and looks up where the customer’s domain (for example, gmail.com or xtra.co.nz) receives mail.
- It connects to the receiving server and hands the message over. The receiving server checks the sender’s authentication records and reputation, then decides whether to deliver to the inbox, the spam folder, or not at all.
- The customer opens the email using their mail app, which reads it with a different protocol (IMAP or POP3). SMTP is only for sending and relaying.
A server that forwards mail on behalf of others is called a relay; our definition of an SMTP relay explains the term.
Ports and encryption
SMTP connections use specific ports. Port 587 with STARTTLS and port 465 with implicit TLS are the usual choices for applications sending through a provider; port 25 is mainly used between mail servers and is often blocked for ordinary connections. Our article on port 465 vs port 587 helps you choose. Mailpro supports encrypted connections with TLS 1.3.
Why not just use your web host’s mail server?
Many small New Zealand websites send through the shared server that came with their hosting plan. It works until it does not: shared servers can have poor reputations because of other customers, sending limits are often low, and there is little or no reporting. A dedicated service offers:
- Better deliverability, because the server is managed for sending and your domain is properly authenticated.
- Visibility, with statistics on delivery, opens and clicks; see SMTP statistics.
- Scalability for busy periods such as Boxing Day sales or the start of the summer season.
- Cost control, with Mailpro pricing based on emails sent.
Authentication: the records that prove it is really you
Receiving servers check three DNS records on your domain: SPF lists the servers allowed to send for it, DKIM adds a cryptographic signature, and DMARC tells receivers what to do if the other checks fail. Set them up for your .co.nz or .nz domain before switching providers. Our FAQ on SMTP server authentication with Mailpro covers the settings.
SMTP, the law and your data
The protocol does not change your legal duties. Marketing emails sent via SMTP still fall under the Unsolicited Electronic Messages Act 2007 (consent, sender identification and a working unsubscribe), while purely transactional messages such as booking confirmations are treated differently. Personal information in those emails is covered by the Privacy Act 2020. Mailpro processes it on private Swiss infrastructure, outside US jurisdiction.
SMTP or API?
SMTP is the quickest option when your software already has email settings. If you are building something new and want delivery status and contact management in code, compare it with an email API in API vs SMTP, or go straight to the Mailpro SMTP server page to get started.