The M-Files flag, shown in Outlook's message list, indicates that you have already saved the message to M-Files.
If some other user (for instance another recipient) has saved the same message to M-Files, M-Files prompts you to confirm whether you wish to save that message again. Even if you do not save the message again, the M-Files flag will now be displayed in Outlook for that message.
With the Update M-Files Status function (in the message's context menu), you can easily display all M-Files flags. Also messages saved in M-Files by other users will then have an M-Files flag in Outlook.