Send WordPress email over SMTP

Before you start

1. Sign in and verify the domain you send from. Viesti accepts a message only when its From address is on a domain verified in your workspace and the envelope sender (MAIL FROM), which most libraries take from the From address, is on the same domain. DNS setup lists the records.
2. Create an API key on the Credentials page of the dashboard. It is shown once. The key is the SMTP password; the username is apikey.

Option 1: WP Mail SMTP

Install and activate WP Mail SMTP from Plugins, then open WP Mail SMTP » Settings and fill in:

In WP Mail SMTP, TLS means STARTTLS. The Auto TLS switch only matters when Encryption is None or SSL.

Keep the key out of the database

Settings entered in the admin are stored in the database and visible to every administrator. WP Mail SMTP also reads them from constants in wp-config.php, placed above the line /* That's all, stop editing! Happy publishing. */; fields set this way are shown grayed out.

define( 'WPMS_ON', true );
define( 'WPMS_MAILER', 'smtp' );
define( 'WPMS_SMTP_HOST', 'smtp.viesti.email' );
define( 'WPMS_SMTP_PORT', 587 );
define( 'WPMS_SSL', 'tls' );
define( 'WPMS_SMTP_AUTH', true );
define( 'WPMS_SMTP_AUTOTLS', true );
define( 'WPMS_SMTP_USER', 'apikey' );
define( 'WPMS_SMTP_PASS', getenv( 'VIESTI_API_KEY' ) );
define( 'WPMS_MAIL_FROM', getenv( 'MAIL_FROM' ) );
define( 'WPMS_MAIL_FROM_FORCE', true );

getenv() returns variables your server passes to PHP, for example env[VIESTI_API_KEY] lines in a PHP-FPM pool; MAIL_FROM is a bare address here. If your host cannot set environment variables, write the values in place of the getenv() calls; wp-config.php is not stored in the database.

Option 2: phpmailer_init, no plugin

Save this as wp-content/mu-plugins/viesti-smtp.php. WordPress loads must-use plugins automatically, without activation.

<?php
/**
 * Plugin Name: Viesti SMTP
 * Description: Sends wp_mail() through Viesti over SMTP with STARTTLS.
 */

add_action( 'phpmailer_init', function ( $phpmailer ) {
	$phpmailer->isSMTP();
	$phpmailer->Host       = 'smtp.viesti.email';
	$phpmailer->Port       = 587;
	$phpmailer->SMTPSecure = 'tls';
	$phpmailer->SMTPAuth   = true;
	$phpmailer->Username   = 'apikey';
	$phpmailer->Password   = getenv( 'VIESTI_API_KEY' );
	$phpmailer->Timeout    = 10;
} );

add_filter( 'wp_mail_from', function () {
	return getenv( 'MAIL_FROM' );
} );

SMTPSecure = 'tls' is STARTTLS in PHPMailer. Without the wp_mail_from filter, WordPress sends from wordpress@ followed by your site's host name without www., which works only if that domain is the one you verified.

Test it

With WP Mail SMTP, open WP Mail SMTP » Tools, Email Test tab. With either option, WP-CLI sends a message and prints the PHPMailer error if it fails:

wp eval 'add_action( "wp_mail_failed", function ( $e ) { echo "FAILED: ", $e->get_error_message(), "\n"; } ); var_dump( wp_mail( "[email protected]", "Your receipt", "Thanks for your order." ) );'

bool(true) means Viesti accepted the message; check Activity in the dashboard.

Port 465

In WP Mail SMTP choose Encryption SSL and SMTP Port 465 (constants: WPMS_SSL 'ssl', WPMS_SMTP_PORT 465). In the snippet set SMTPSecure = 'ssl' and Port = 465.

Common errors

Replies from Viesti

Whatever the library, its error includes the server's reply. These are the ones caused by your setup or account:

A 250 reply means Viesti accepted the message. Content screening runs before relay; mail it flags is held for review and shows as quarantined on the Activity page.

Next steps

• DNS setup: DKIM, SPF, DMARC and return-path records for your domain.
• SMTP reference: ports, TLS and authentication in detail.
• HTTP API: send the same message as JSON over HTTPS.
• Limits and Bounces: quotas, message size and suppressions.

Plugin fields are from WP Mail SMTP's Other SMTP guide and constants guide, constant names from the plugin's source (version 4.9.0); hooks from the WordPress reference for phpmailer_init and wp_mail_from. Tested with WordPress 7.1.2. Checked 26 September 2026.