As @BjarniRunar stated, Mailpile is not a mailserver. You will actually need a mailserver in order to sent/receive emails. And it might be hard to grasp, but Mailpile isn’t a webmail application. It’s merely a mail client with the focus on security, privacy and encryption.
Mailpile supports any IMAP/SMTP mailbox (and then some). Bluntly said: it doesn’t care which domain you use. If you are able to get a mailbox on nsa.gov - you can even use that one 
You can use the machine you are referring to as a mailserver. But hosting a mailserver is a completely different game than running a Plex server. Eg, for example:
- A mailserver benefits more from stable and reliable hardware, than a hefty amount of resources (like Plex requires).
- Loss of connectivity (whether due to a power outage or network issues) do directly impact your mail and mailbox.
- It is recommended to run your mailserver on a static IP, where you are able to set a reverse DNS. Mailservers with dynamic IP’s can cause nightmares by the dozen.
Are you willing to run your own mailserver or perhaps look for a less expensive mailbox provider?