Why would you want a mail specific stack of hosting, storage, indexing and frontends? If it’s all plain text anyway so the regular storage solutions for files come a long way.

Because email has metadata. From, to, sent date, subject, etc. Plus attachments that may be binary.

Pdf? You converted plain text to something designed to preserve formatting? But why?

You could use maildir and find things with “grep” or any mail client like Thunderbird.

“personalized”

Raspberry Pi is not a server. That people use it as one does not mean it’s fit for purpose.