Could you please allow the plugin a 3rd option to be able to send via "WP Mail"? This would be the absolute best way and also a nice feature for any of your plugins that send mail (i.e. smtp, php, or wp mail). If you really wanted it to be even greater to allow the user to declare a order of fail safes would be even better (i.e. if wp mail fails, use php, if php fails us smtp etc or whatever order the user wanted).
eNewsletter plugin does not use the WordPress mail function to send mail. By using PHP mail directly, it is not compatible with either Postman or Mandrill because of this. Could you please at least allow the plugin (any any of your other plugins that send mail especially Membership2 Pro or other key plugins . . . but preferably any other plugins you have that send mail) make it WordPress mail-compatible?