SMTP stands for Simple Mail Transfer Protocol. It is the standard set of rules that email servers use to pass messages to each other, and it has been the backbone of email since the early 1980s. Every email you have ever sent, from a Gmail message to an invoice from your accounting software, travelled at least part of its journey by SMTP. Our glossary has a shorter definition of SMTP.

How an email travels

  1. Your software hands the message over. Your email program, website or accounting package connects to an SMTP server and says: here is a message from this sender to this recipient.
  2. The sending server checks and queues it. A good SMTP service checks that you are allowed to send, signs the message with DKIM and queues it.
  3. It looks up the destination. The server asks the DNS where mail for the recipient’s domain, such as outlook.com or a company domain, should go.
  4. The receiving server decides. It checks SPF, DKIM and DMARC, looks at the sender’s reputation and then delivers to the inbox, sends to junk or rejects.
  5. The recipient reads it using IMAP, POP3 or a webmail interface, which are separate protocols for collecting mail.

Ports in plain English

  • 587: the usual choice for software submitting email, with encryption started through STARTTLS.
  • 465: SMTP over an encrypted connection from the start. Also widely supported.
  • 25: used between mail servers. Many internet providers and cloud hosts block outgoing connections on it from customer machines, which is why your software may fail if it is set to port 25.

Our blog compares port 465 and port 587 if you need to choose.

Why not just send from your office mailbox?

Microsoft 365 and Google Workspace mailboxes are designed for people, not systems. They have daily sending limits, they don’t give you delivery statistics, and a surge of automated invoices can get the account throttled. A dedicated relay is built for volume, tracks every message and keeps your staff mailboxes separate from system email. That separation protects your day-to-day email if something goes wrong with a bulk send.

What makes an SMTP service trustworthy

  • Improved deliverability: authenticated sending and a well-managed reputation keep messages out of junk folders.
  • Greater control: logs and statistics show what happened to each message. See SMTP statistics.
  • Scalability: a relay handles monthly invoice runs and Black Friday order spikes without you managing servers.
  • Cost control: Mailpro charges by sends, not contacts.
  • Security: Mailpro encrypts connections with TLS 1.3 and hosts data on private Swiss infrastructure, outside US jurisdiction.
  • SMTP relay: a service that accepts mail from your software and delivers it onwards, like Mailpro’s. See the SMTP relay definition.
  • MTA (mail transfer agent): the server software that moves mail between systems using SMTP.
  • Bounce: a message returned as undeliverable. Hard bounces are permanent; soft bounces are temporary.
  • Envelope sender vs From address: the technical return address used for bounces can differ from the name your recipient sees, which is one reason SPF and DMARC exist.

SMTP and UK data protection

An SMTP provider handles the content of every message it relays, including names, addresses and sometimes financial details. Under UK GDPR it acts as your processor, so you need a written agreement and should know where it processes data. Switzerland is recognised by the UK as adequate, so a Swiss relay keeps that question simple. Whether a message counts as marketing under PECR depends on its content, not on the protocol.

Ready to connect your systems? See Mailpro’s SMTP server for UK businesses, or read how to check an SMTP connection if your software refuses to send.

Reliable SMTP for your the UK applications

Route transactional email through Mailpro’s Swiss SMTP relay and track every message. Start free with 500 credits.

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