Mail Window not formatted as it used to be
I have just updated to Venture and when I work in Mail the format of the window had changed to one I don't like. Mailboxes in panel on lhs of screen, my messages in date order in the central panel and the body of the email I have selected in the third largest panel. I prefer this top down rather than left to right but can't seem to find a way to reformat the window. Today I seemed to click on something within the mail window accidentally and the panel with the text of my selected email has disappeared and I now just have mail boxes and a list of my incoming mails in date order. I have to click on a message to open a new window in order to read it now and this is not very efficient. I read somewhere of other people having the same issue but not been offered a solution. Would love to go back to left hand side panel with inboxes etc, then a list of my mails at the top of the window in an expandable panel and below that a panel featuring the email I wish to read and respond to.
Thank you in anticipation of a simple fix that I am missing.
James