Page "Microsoft Mail" Paragraph 7

A single postoffice was locked to a limit of 500 mailboxes.

This meant a large enterprise would require many postoffices and many MTAs to connect these postoffices.

Also, the Microsoft Mail Client for Windows used an MMF file to store messages, they were downloaded off the postoffice.

Because of the file lock and the way they were accessed a user could only login to their mail from a single PC at a time.

This made using MSMail across multiple machines problematic.