I recently came across an issue on an Ubuntu 20.04 installation recently when I setup Postfix using a smart relay with authentication. But when mails were sent, the following error was being recorded:
warning: SASL authentication failure: No worthy mechs found
I did some research and found I needed to install the libsas12-modules. I though I would share this incase you find it useful.
sudo apt install libsasl2-modules