SMTP API

Connecting to Paubox Email API via SMTP

To configure your application or service to send email through Paubox’s SMTP relay to Email API, follow these steps:

  1. Generate an API Key

  2. Configure the SMTP server host to smtp.paubox.com.
    This is often labeled as the SMTP relay or outgoing mail server in most email clients.

  3. Set your username to the literal string apikey.
    Note: this should be the exact word "apikey", not the API key you generated.

  4. Use your API key as the SMTP password.

  5. Choose the appropriate port, typically 587, unless your network requires another.

ℹ️ Note
If you're pasting a base64-encoded API key, double-check for any extra spaces or line breaks.
These can inadvertently appear when copying from certain terminals or editors, and they will prevent authentication.
Since SMTP is a line-based protocol, even a stray newline can result in failure.

Available SMTP Ports

Choose the correct port based on your desired level of security:

  • For TLS connections, use:
    25 or 587

  • For SSL/TLS connections, use:
    465

ℹ️ Tip
If you're not sure which one to use, 587 with STARTTLS is the most widely supported and recommended option.

Sending Mail via SMTP

Once your SMTP connection is set up, you’re ready to construct and send email messages using your preferred client, service, or library.

Rate Limits

To avoid delivery interruptions, be aware of Paubox's SMTP usage thresholds:

  • A single IP address may send up to 500 messages per minute.

Avoid Direct IP Usage

Always reference the host as smtp.paubox.com — do not use direct IP addresses.
Paubox’s infrastructure may change without warning, and relying on hardcoded IPs could cause unexpected delivery issues in the future.