Temporary forwarding mail-server

gistfile1.txt

docker run -p 25:25 --name postfix -d catatnight/postfix

docker exec -it postfix bash

apt-get install postfix-pcre

in /etc/postfix/main.cf
correct myhostname 
smtpd_relay_restrictions = check_relay_domains
relay_domains = *list of accepting hosts*
virtual_alias_maps = pcre:/etc/postfix/virtual-regexp

in /etc/postfix/virtual-regexp
/.*@.*/ forwarding@mail.de

To get the relay_domain to accept, in /etc/postfix/master.cf
smtp       inet  n       -       n       -       -       smtpd -v <-- the -v is new

and tail -f /var/log/mail.log (there you can see the host when it is trying to send)