Sending Transactional Email with ExpressionEngine using Dotmailer

config.php

$config['email_crlf']       = "\r\n";
$config['email_newline']    = "\r\n";
$config['mail_protocol']    = 'smtp';
//The server may need to change depending on the region assigned in your Dotmailer account
//https://support.dotmailer.com/hc/en-gb/articles/212214408-Using-transactional-email#viasmtp
$config['smtp_server']      = 'r1-smtp.dotmailer.com';
$config['smtp_username']    = 'TRANSACTIONAL_EMAIL_USER';
$config['smtp_password']    = 'TRANSACTIONAL_EMAIL_PASSWORD';
$config['smtp_port']        = '25';
$config['email_smtp_port']  = '25';