A friend received a notice from yahoo saying their "briefcase" mail will be deleted in a few weeks what is bri?

The note I received from a friend said their "briefcase" would be deleted along with their email unless they downloaded it - I do not believe this will affect regular yahoo email (in box mail, etc) - will it? As far as I know, I do not use a "briefcase" on yahoo - anyone that can help me? Thanks, Tom