MailPoet SMTP Configuration

MailPoet does not use wp_mail to send mail, so there’s no way Postman can manage it. The MailPoet people prefer to build their own SMTP handling right into the plugin. There are other newsletters that are compatible with Postman. See Postman’s description page under Known Compatibility.

See https://wordpress.org/tags/postman-mailpoet and especially MailPoet’s response here https://wordpress.org/support/topic/integration-with-postman-smtp?replies=9