Make WordPress use an SMTP server for all outgoing email notifications.

wordpress-smtp.php

<?php

class mdgSMTP {

	public static function setup() {

		add_action( 'phpmailer_init', [ __CLASS__, 'configure_php_mailer' ] );

		if ( class_exists('WP_CLI') ) {
			WP_CLI::add_command( 'send-test-email', [ __CLASS__, 'send_test_email' ] );
		}

	}

	public static function configure_php_mailer( $phpmailer ){

		if ( !defined('SMTP_USERNAME') || !defined('SMTP_PASSWORD') ) return;

		// MESSAGE
		$phpmailer->FromName = '…';
		$phpmailer->From     = '…@…';

		// SERVER
		$phpmailer->isSMTP();
		$phpmailer->Host = '…';
		$phpmailer->Port = 2525; // WP Engine does not allow port 25. Allowed ports: 2525 (recommended), 587 (required for Microsoft), or 465.

		// USER
		$phpmailer->SMTPAuth = true;
		$phpmailer->Username = SMTP_USERNAME; // Set these constants in wp-config.php, e.g.: define('SMTP_USERNAME', '…');
		$phpmailer->Password = SMTP_PASSWORD; // Set these constants in wp-config.php, e.g.: define('SMTP_PASSWORD', '…');

		// SECURITY
		$phpmailer->SMTPAutoTLS = true;

	}

	public static function send_test_email( $args ) {

		$email = $args[0];

		WP_CLI::line( 'Sending test email to: ' . $email );

		wp_mail(
			$email,
			'Testing SMTP Email from ' . get_bloginfo('name'),
			'This email should have been sent from WordPress via the configured SMTP server.'
		);

	}

}