To speed up disk I/O and overall IMail performance, IMail has the ability to mark a message as deleted so that it no longer appears in an email client, but not to rewrite the mailbox file on the server every time a message is deleted.

This setting is controlled by a value named 'MBXCompress' under: HKEY_LOCAL_MACHINE\SOFTWARE\Ipswitch\IMail\Global
in the registry. When the mailbox has X percent wasted space, the mailbox will be compressed. The default is 10 percent. Unless this value is created in the registry with a different setting, 10 percent will be used.

If you wish to adjust this value, create MBXCompress as a DWORD value under the Global key. Set it to the percentage at which you want the mailbox to be compressed. 0-100 (decimal) are valid entries for this value. The mailbox is compressed by removing all messages that have been marked as deleted. If you set it to 0 this will effectively turn off the mailbox compression feature.