install stunnel4
apt update
apt install stunnel
Create certificate file from Lets Encrypt
vim /root/renew-cert.sh
(cat /etc/letsencrypt/live/wiki.lexodexo.de/privkey.pem ; echo ""; cat /etc/letsencrypt/live/wiki.lexodexo.de/cert.pem; echo "") > /etc/stunnel/stunnel.pem
configuring incoming IMAP traffic
vim /etc/stunnel/ssl_in.conf
; /etc/stunnel/ssl_in.conf
; Certificate/key is needed in server mode and optional in client mode
cert = /etc/stunnel/stunnel.pem
; Some security enhancements for UNIX systems - comment them out on Win32
chroot = /var/lib/stunnel4/
setuid = stunnel4
setgid = stunnel4
; PID is created inside chroot jail
pid = /srv1.pid
debug = 4
;output = /tmp/incoming.log
; Some performance tunings
socket = l:TCP_NODELAY=1
socket = r:TCP_NODELAY=1
[ssl_in_imap]
accept = 993
connect = localhost:10993