where one can read:
in /etc/dovecot/conf.d/10-ssl.conf add
ssl_cert = </etc/path/to/your/cert.pem
ssl_key = </etc/path/to/your/key.pem
in /etc/postfix/main.cf add
smtpd_tls_cert_file = /etc/path/to/your/cert.pem
smtpd_tls_key_file = /etc/path/to/your/key.pem
but I still have a general additional question:
I have several virtual domains (domain1.tld, domain2.tld, domain3.tld, …) served by Apache webserver for the same IP XY and each domain has its own /etc/path/to/your/cert.perm or key.perm (i.e. on my server /usr/local/etc/letsencrypt/archive/domain1.tld).
So I wonder which one of the different cert.perm/key.perm I have to use in main.cf and 10-ssl.conf?
Because for all different domain1.tld, domain2.tld, domain3.tld reachable under IP XY there is a MX-Record with IP XY so that later one should be able to use smtp.domain1.tld, smtp.domain2.tld, smtp.domain3.tld etc. in the email client.
Thanks and best regards