Automatic folder compaction and #PB# files
In order to reclaim unused disk space, Postbox may compact the per folder "mbox" files that store your email. Because folder compaction is a resource-intensive operation, Postbox does this when left idle.
By default, Postbox will only attempt to compact a folder if it will save 100MB of disk space. This preference can be changed in Preferences | Options > Advanced > Network & Disk Space.
When compacting local mail folders (including POP accounts), Postbox will keep a backup of the original folder alongside the compacted version. For example, if the folder is Inbox, the on-disk backup will be named #PB#Inbox. Only one backup (per folder) is kept around at any time. It is safe to delete these #PB# files.
Note that backup files are not made for folders on IMAP accounts (as the server has a copy of the messages.)
To manually compact folders, please select Compact Folders from the File menu.