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Outlook on the web (previously known as Exchange Web Connect, Outlook Web Access, and Outlook Web App) is a personal information manager web app from Microsoft.It includes a web-based email client, a calendar tool, a contact manager, and a task manager.

Hiri was (now defunct) a business focused desktop e-mail client for sending and receiving e-mails, managing calendars, contacts, and tasks. It was developed as an alternative to existing e-mail clients and calendar applications such as Microsoft Outlook and Mozilla Thunderbird.

The limit became 20 GB for Outlook 2003-2007, and increased to 50 GB from Outlook 2010. A file that is created in the personal-folders format in Outlook 2003 or later is not compatible and cannot be opened by earlier versions. As with any file, .pst files can become corrupted.

MAPI also had a service provider interface of sorts. Microsoft used this to interface MS Mail to an email system based on Xenix, for internal use. Extended MAPI is the main e-mail data access method used by Outlook, to interface to Microsoft Exchange, via MAPI service providers shipped with Outlook. MAPI/RPC protocol details