The main difference between SMTP and MAPI is the type of emails that you can use them with. If you would like to send and receive emails, MAPI is the best option for you since you know that it will be able to access your inbox and your outbox easily.
Using SMTP will only allow you to send emails but not receive them. What they stand for are also different. When you say MAPI, this stands for Messaging Application Programming Interface while SMTP stands for Simple Mail Transfer Protocol. You need to understand the different email protocols so that you will be able to choose the best one for your needs.