Hey John, 

Thanks for reaching out regarding this question. Mail Enable is another email server software. If you already have SmarterMail installed then you should DEFINITELY uninstall this. Having both running at the same time could cause port conflicts. Or Mail Enable could respond and say that a user on your SmarterMail user doesn't exist causing confusion between the sender and the recipient.  

Please let me know if this helps. 

Kind Regards,