Mac LDF: Local Mail File Compression

Version 3.1 of Microsoft Mail for AppleTalk Networks includes a feature that allows users to store mail in local folders. Mail stored in local folders is actually written to a local mail file which resides in the System Folder:Preferences:MS Mail Local Folders on the local hard disk. A local mail file is created for each user who logs into Mail from a particular machine and is named after the Mail user.

When this file is not in use by Mail to save or read messages, the local mail file is in a state of compression. In Mail 3.1, the compression occurs whenever there is empty space in the local mail file. Version 3.1 of the workstation software checks the file approximately every 15 seconds for space to compress. This can result in problems for PowerBook users, because the consistent disk access does not allow the machine to "sleep." This problem was addressed through a new compression routine introduced in Mail 3.1a.

Mail 3.1a only performs the compression routine when the local mail file is accessed to save or read a message. Therefore, with Mail 3.1a, compression is not occurring as a background process and will allow PowerBooks to "sleep."