Fix SSL and PostFix on a OSX server

Hi,
I have tried to set up Lets Encript using this Tuto :
https://www.macstrategy.com/article.php?211

I got stuck at the last part
serveur:~ miniserve$ sudo security import /etc/letsencrypt/live/cnsr.clubiwan.fr/letsencrypt_sslcert.p12 -f pkcs12 -k /Library/Keychains/System.keychain -P "Password" -T /Applications/Server.app/Contents/ServerRoot/System/Library/CoreServices/ServerManagerDaemon.bundle/Contents/MacOS/servermgrd

security: SecTrustedApplicationCreateFromPath /Applications/Server.app/Contents/ServerRoot/System/Library/CoreServices/ServerManagerDaemon.bundle/Contents/MacOS/servermgrd: UNIX[No such file or directory]

I d'ont have the OSX server, so I d'ont have the Server.app
So I'm stuck at : Import the certificate into the OS X / macOS Keychain
sudo security import /etc/letsencrypt/live/your_domain_name.tld/letsencrypt_sslcert.p12 -f pkcs12 -k /Library/Keychains/System.keychain -P "admin_password" -T /Applications/Server.app/Contents/ServerRoot/System/Library/CoreServices/ServerManagerDaemon.bundle/Contents/MacOS/servermgrd

Second part is Post Fix, It is running, It was previously well setup for an other domain.
I need it to run and start at system reboot and fix all the DKIM, SPF, DMARK etc...

I will provide DNS on private message