Apple Mail Since upgrade to dovecot 2.3.x unable to connect

[Gregory Heytings]

>
> ssl_cert = </etc/ssl/letsencrypt/idaweb-mail.rooot.de/fullchain.pem
> ssl_key = </etc/ssl/letsencrypt/idaweb-mail.rooot.de/key.pem
> ssl_ca = </etc/ssl/letsencrypt/idaweb-mail.rooot.de/ca.pem
>

This is wrong, it should be:

ssl_cert = </etc/letsencrypt/live/idaweb-mail.rooot.de/fullchain.pem
ssl_key = </etc/letsencrypt/live/idaweb-mail.rooot.de/privkey.pem

The address idaweb-mail.rooot.de does not resolve.  There is a 
webmail.rooot.de , but its certificate is for mail.rooot.de , which is 
wrong.  There is also a mail.rooot.de , whose certificate is also for 
mail.rooot.de , which is okay.

Yet another possibility (but it seems less likely given that an Apple Mail 
from 2016 is a reasonably recent mail client) is that it does not support 
recent enough SSL protocols, which were enforced by your server upgrade. 
See the entries for MinProtocol and CipherString in the openssl.cnf file 
on the server.

Gregory