When you try to use Messaging API (MAPI) to set up Microsoft Office Outlook 2003 to connect to Microsoft Exchange Online in a Microsoft Office 365 environment, the MAPI connection might not work.
Outlook 2003 isn't a supported email client for connecting to Exchange Online mailboxes by using MAPI in Office 365. Note
Outlook 2003 is supported for connecting to Exchange Online mailboxes by using the Post Office Protocol (POP) or Internet Message Access Protocol (IMAP) protocols.
For more information about the system requirements for Office 365, go to the following Microsoft websites:
Still need help? Go to the Office 365 Community