my column of mailboxes is missing, and the "View" tab does not help me

If you were to drag the column separator all the way to the left the mailbox list will disappear, and there is no obvious way to restore it.

The View menu (not a tab) > Show Mailbox List should restore it.

I'm not the original poster, but I wanted to say this solution (dragging the panel open) worked for me. Just note, as you said, there is no obvious bar to drag, but I dragged the edge of the window to the left and it opened the panel, so maybe that will work for others as well. Thanks!

Glad you figured it out.

Regarding that specific concern, I posted a much better description here: Where can I find the "Horizontal Splitter" in Apple Mail? - Apple Community