I have found a simple solution for using letsencrypt with exim and dovecot.
1) edit /etc/exim.conf
#tls_certificate = /etc/exim.cert
#tls_privatekey = /etc/exim.key
tls_privatekey = ${if exists{/etc/exim.key.${tls_sni}}{/etc/exim.key.${tls_sni}}{/etc/exim.key}}
tls_certificate = ${if...