SMTP (Simple Mail Transfer Protocol) is the language servers and apps use to send email. Receiving mail uses IMAP or POP3; sending always uses SMTP.
Two SMTP journeys
- Submission: your app sends the message to your outgoing server,
mail.yourdomain.co.uk, logging in first. - Relay: your server delivers it to the recipient’s server.
Why authentication is required
An SMTP server that accepted mail from anyone would be used by spammers within hours. SMTP authentication means your app proves who it is — using the full email address and mailbox password — before the server will send anything.
In app settings this appears as:
- “My outgoing server requires authentication” — tick it
- “Use same settings as incoming server” — usually correct
- “Require sign-in” — tick it
SMTP from websites
Websites send mail too: contact forms, WordPress notifications, shop receipts. They can hand mail to the server’s local mail system, or log in to a mailbox over SMTP like an app. Authenticated SMTP is far more reliable. Sending email from your website reliably
Ports
| Port | Use |
|---|---|
| 465 | Submission with SSL/TLS (recommended) |
| 587 | Submission with STARTTLS |
| 25 | Server-to-server relay; often blocked for apps by ISPs |
Email ports and encryption: SSL/TLS vs STARTTLS explains the security options.