In apple Mail: bold names of email accounts at top of left window disappeared.

During an update awhile ago of OS, the appearance of the left window in Mail changed. My several email account names in bold were there, and the names in smaller pale letters were also placed at the bottom of the window under the heading “on my Mac.” Today, for some unknown reason, the bold account names are gone, and I only have the ones at the bottom of the left screen. How can I get my bold account names to re-appear at the top of the window? Here’s a picture of what I have now:

[Edited by Moderator]