You are mixing a lot of other things together.
Its mandatory to understand what is postfix and what is roundcube.
Postfix is MTA, aka Mail Transfer Agent, and this is truthfully SMTP server, which is SENDING and RECEIVING emails from other SMTP servers.
Roundcube is on the other hand MUA, which stands for Mail User Agent, it acts as a mediator between you, as a user, and your SMTP server. MUA is also Outlook for example, which will let you type emails and read them, as well as you can in Roundcube.
So the Scheme is something like this:
USER → MUA (Roundcube / Outlook) → MTA (Postfix) → — ETHERNET — → MTA → MUA
Virtualmin have built in MUA client called Usermin, which lets you browse and type emails as well as Roundcube or Outlook.
Alternatively, you can install Roundcube, and use it instead of Usermin.
So to answer your question - When you sucesfully installed Virtualmin, postfix is practically good to go, it works out of the box, you just have to configure afew things like SMTP server hostname, SPF, DKIM, DMARC, rDNS, and also dont forget to check A and MX records for your domain. Dont get me wrong, sending and receiving emails will actually most probably work (receiving will work only with good A and MX records), but mailservers which are receiving your emails will most probably block you right away.
This is a more complex topic that I cannot fully cover here, but you have got those shortcuts that I write above, just use Google or ChatGPT, and you will find everything you need to set it up - it is not that much complex when you invest afew hours of your time.
btw, thanks for correction Joe