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Send via SMTP

In addition to our RESTful API, MX18 also supports traditional SMTP for sending emails — ideal for plug-and-play integrations with third-party email clients, marketing platforms, and backend applications.

Pre-requisites

Before sending via SMTP, make sure:

  • You have an active MX18 account – Sign up here
  • You have your SMTP credentials (your MX18 email and API token)
  • Your sending domain or email is verified under your account

What is SMTP?

SMTP (Simple Mail Transfer Protocol) is a standard communication protocol used to send emails from one server to another.

SMTP vs API

Feature SMTP API
Setup Minimal, works with email clients Requires HTTP integration
Flexibility Limited customization Full control over headers, payload
Use case Basic transactional emails Scalable, templated, programmatic use
Event tracking Limited Rich, real-time event tracking

Use SMTP if you want a quick and universal way to send. Use the API for more advanced workflows, templates, and personalization.

Sending with SMTP

Configure your email client or application using the following credentials:

Field Value
SMTP Server smtp.mx18.com
Port (TLS) 587
Username Your MX18 registered username
Password Your API token
Authentication LOGIN or PLAIN
Encryption TLS

Ensure that your email sender domain is verified in MX18 before sending.

Send SMTP via telnet (for testing)

You can test MX18's SMTP using Telnet:

telnet smtp.mx18.com 587

Then run: 

EHLO yourdomain.com
AUTH LOGIN
(base64 encoded username)
(base64 encoded API token)
MAIL FROM:<info@yourdomain.com>
RCPT TO:<user@example.com>
DATA
Subject: Test from Telnet

Hello, this is a test email via Telnet.
.
QUIT

Use https://www.base64encode.org to encode your credentials.

Common SMTP error codes

Code Meaning Suggestion
421 Service not available Try again later, check server availability
450 Mailbox unavailable Recipient email may be temporarily unavailable
451 Requested action aborted: local error Server-side issue — retry after some time
452 Insufficient system storage Reduce message size or check recipient quota
500 Syntax error, command unrecognized Check SMTP syntax
530 Authentication required Ensure valid username/token is used
535 Authentication failed Verify base64 token or login format
550 Requested action not taken: mailbox unavailable Invalid or unverified recipient
553 Invalid sender address Ensure sender identity is verified in MX18

Need help?

  • Confirm your SMTP username and token are correct

  • Ensure your sending domain is verified

  • Try sending from the dashboard to test connectivity

If you're still facing issues, contact us at support@mx18.com — we’ll get you unblocked fast!