The basic problem is that without an MTA, these mailboxes have no email address. They are nothing but some files on disk. Dovecot doesn't need to understand email addresses.
The usual arrangement between an MTA (like postfix) and Dovecot is that both the MTA and Dovecot are configured to expect the same layout of either mbox files or maildir directories. But the mapping is very different for the MTA vs Dovecot.
- Dovecot understands which users read / write which mailboxes
- The MTA understands which email addresses get routed to which mail boxes.
You might assume that an email sent to [email protected] would go to user bob; that only happens because the MTA knows to route [email protected] into bob's mailbox /home/bob/.mail. Then when bob logs in via IMAP or POP3, Dovecot know that bob's mailbox is /home/bob/.mail.
...But the MTA might also route emails for [email protected] to bob's mailbox too. It may even route [email protected] into /home/mary/.mail when bob quits the company and his successor receives his email. It's entirely the job of the MTA to decide.
So without an MTA of some sort, you cannot route email into inboxes.
There are simpler MTAs. Postfix is rather awkward to configure and nobody wants the risk of accidentally misconfiguring an MTA and letting spammers abuse it.
I'll draw your attention to another question here on U&L which is similar (not duplicate?). Minimal MTA that delivers mail locally (for cron)?
If you are willing to roll your own, then the formats of both maildir and mbox are pretty simple. Though maildir is much safer to work with. Delivering to maildir is as simple as writing to the tmp directory and then moving (renaming) it into the new directory.
Of course these have to be well formatted emails with appropriate headers such as From: To: Subject: Date:. Examples can be found in RFC 5322