Thank goodness, it works!!! I no longer get the error regarding the failed forward to gmail.
I don't know how to install a fresh clone other than to run git clone firstname.lastname@example.org:mail-in-a-box/mailinabox.git
which I have already tried.
Pity about the Let's Encrypt but I suppose I shall just have to wait for the TLS update. Now, back to the Rainloop errors.