Within a very short time of loading the Postie plugin - off the WordPress.org plugin site I was contacted by the web host to say that they had detected a lot of emails sent from the site (It was a clean new WP site). They discovered the Postie plugin had sent approximately 70K messages. (darn - I hate to have contributed to that).
I had found the plugin via some reviews (thought they were safe ones).
I cannot recommend using this plugin and I suggest anyone using it should be cautious and check it has not been compromised.