MAPI cannot be used on AIX directly. To use MAPI on AIX box you need to provision a Windows box and install Email Engine there pointing to your AR Server on AIX box. After that you can create an user with Exchange mail profile and use it for outgoing and incoming messages. This way you can use MAPI with AIX.
I may be wrong but I don't think you can use MBOX protocol directly with Exchange Server.
If you don't want to use MAPI the above way and since POP3 is disabled in your environment, you can use IMAP4.
To use IMAP4 for incoming messages, make sure it is enabled on your Exchange Server for user you will be using to recieve mails.
For more information on how to configure Incoming Mail Box and create requests using incoming emails, please refer to 7.1 Email Engine guide.