How do i get certbot to work behind nginx-proxy-manager for my mail server ?

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I have a mail server using letsencrypt certificate for imaps. But i cant figure out how to setup nginx-proxy so the mail server is allowed to renew certificat beind nginx-proxy. I have searched hig an low but i cant figuer it out.

Every time i need to renew the certificat i port forward direct to the server. Is there a way to not doing that. ? if this is not the the right place please point me in the right directions.