Not sure what you mean by "second domain" but here's how you'd secure Nginx with Let's Encrypt:
Hi pnjo23
Sorry if my description was lagging some info.
I have created my mailserver with domain1.com and added certificates according to the guide you have described. Everything is working perfectly.
Now I would like to add a second domain, let's call it domain2.com. In order for me to allow users to access both domain1.com/mail and domain2.com/mail I need to add SSL certificates for domain2.com in Nginx somewhere, but I can't seem to figure out how to do it 
For Postfix and Dovecot I assume I do not need to add a new certificate, since I have added domain1.com as MX for domain2.com, and therefore only uses domain1.com for mails.
I'm aware I could just make a webforward or something from domain2.com to domain1.com, but I think the other way around is more correct.
I found this guide: https://www.linuxbabe.com/mail-server/u … stallation - See the last bit about "Using Separate Domain for RoundCube". It is this part I'm struggling with.