slocal would drop my mail into /var/mail/spool/kevin from which I could grab it with inc. My setup was more complicated than that actually, as I used spamassassin. My .maildelivery, used by slocal, would filter expected spam into a spam spool file. I also had email from lists such as this tagged and deposited into a list spool file.
I have a ~/.xmhcheck file containing (without indentation)
Something in xmh, exmh, or inc would grab mail from those spools and deposit it into the right email folders as numbered messages. I've lost track of how that worked.
The email server machine that I ran for 10-15 years died about 18 months ago, and I've only partially resurrected what I used to do, as I want to get on board with all the new fangled email security goodies out there for servers, e.g. TLS. Why that matters to you is that I don't have a complete picture for you to get this working using our preexisting mail tools. Maybe someone else will be able to help fill in the gaps?
Best-o-luck, and stay healthy!