By the way, I would use reloadcmd instead of posthook so in your case, if you are not installing your certs to any other dir, this command will have the same effect, restart your services once you get a certificate for your domain.
/root/.acme.sh/acme.sh --install-cert -d mydomain.de --reloadcmd "systemctl restart apache2 postfix dovecot" --cert-home /etc/acme
Note: Keep in mind I recommend the use of reload instead of restart.