Raymond,

> # certbot certonly --apache

> Saving debug log to /var/log/letsencrypt/letsencrypt.log
> Could not choose appropriate plugin: The requested apache plugin does not 
> appear to be installed
> The requested apache plugin does not appear to be installed
> I am trying to find which is the "requested apache plugin". The log mentions 
> "authenticator".

just wondering: Do you have an apache webserver running on this machine?
If not, run

# certbot certonly --standalone

or read https://certbot.eff.org/docs/using.html

and don't forget to open port 80 in your firewall. letsencrypt wants to access 
that port.

> I am hoping that an Apache Certificate will be useful for Dovecot.

you can bet :-)

Michael

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