What mailbox (file) formats does bogofilter understand?
Bogofilter understands the traditional Unix mbox format, the Maildir and MH formats. Note though that bogofilter does not support subfolders, you will have to explicitly list them in MH or Maildir++ folders – just mention the full path to the subfolder. For unsupported formats, you will have to convert the mailbox to a format bogofilter understands. Mbox is often convenient because it can be piped into bogofilter.