I figured it out.

Mark Gilbert wrote:
We thought we'd try the latest Eudora. Apparently it is now done by
Mozilla. Didn't like it and uninstalled. But now there is a new column
in the email headers in TB which indicates the word "read" for every
email that has been read. And when we minimize TB then maximize it . .
. it says we're looking at Eudora.

Help! How do we get rid of this.

TIA