Do not use the mail icon for new messages
This is bad as this is an icon for mail (apps) and is used by some popular ones, e.g. K9 mail. As Briar is not a mail app, you should not use this, otherwise users get confused, because they think they have a new mail, while they actually have a new Briar message.
Better use the icon (or a variation) of Briar itself. This makes clear that there are new Briar messages.