Installing postfix + dovecot

I am following this tutorial to install a complete mail server on my private server:

I am at the point I need to generate certificates with sudo certbot --nginx.
Since I have already installed and configured my website using caddy, I was wondering

  • MUST/SHOULD I use certificates generated by caddy for my email server? Is it mandatory?
  • If so, Postfix is waiting for pem files (fullchain.pem and privkey.pem ), while caddy is generating (as far as I understood) a cert file and a key file. Any hint for the transformation? Can I use the steps in ?
  • And as far as I understood the certificates get renewed regularly. How can I know when it occured?

Sorry if the questions looks stupid or dumb. I am completely new to this kind of problematic, and I never setup a mail server neither a web server.