Most entries under this key affect automatic compression of the Mail message file, which by default has the filename extension of .MMF. When enabled, automatic compression uses idle time on your PC to recover disk space freed by the deleted messages and returns the disk space to the file system. You should not need to change the default values for entries in this subkey.
This is the Registry path for this subkey:
HKEY_CURRENT_USER\Software\Microsoft\Mail\MMFKb_Free_Start_Compress REG_SZ kilobytes
Background compression starts when at least this much recoverable space is detected in your message file. Both Percent_Free_Start_Compress and this entry are always active. The first entry to trigger starts the compression.
Default: 300
Kb_Free_Stop_Compress REG_SZ kilobytes
Background compression stops when there is less than the indicated amount of recoverable space in your message file. This avoids the unnecessary difficulty in trying to recover the last little bit of free space. Both this entry and Percent_Free_Stop_Compress are always active. The first entry to trigger stops the compression.
Default: 100.
No_Compress REG_SZ 0 or 1
Specifies whether background compression is to be disabled. A value of 1 disables background compression of the .MMF message store.
Default: 0 (That is, background compression is enabled.)
Percent_Free_Start_Compress REG_SZ percent
Background compression starts when the amount of recoverable space rises above this percentage of the total file size. Both Kb_Free_Start_Compress and this entry are always active. The first one to trigger starts the compression.
Default: 10
Percent_Free_Stop_Compress REG_SZ percent
Background compression stops when the amount of recoverable space falls below this percentage of the total .MMF file size. Both this entry and Kb_Free_Stop_Compress are always active. The last one to trigger stops the compression.
Default: 5
Secs_Till_Fast_Compress REG_SZ seconds
The background compression algorithm has a fast mode and a slow mode. Background compression begins in the slow mode to avoid slowing system response time. After a number of seconds of system inactivity indicated by this entry, the compression switches to fast mode. Any user activity changes the setting back to slow mode.
Default: 600 seconds (That is, ten minutes of system inactivity.)
See also the entry for AppInit_DLLs in "Windows Software Registration Entries."