Dovecot enable SSL (by default it uses an old expired cert if you choose pop3s and imaps as protocols)
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Create Cert & Key:

openssl req -new -x509 -nodes -days 1530 -out server.crt -keyout server.key
mkdir /etc/mailssl
chmod 700 /etc/mailssl
cp server.* /etc/mailssl

Edit /etc/dovecot.conf

ssl_cert_file = /etc/mailssl/server.crt
ssl_key_file = /etc/mailssl/server.key