I believe you are talking about S/Mime here. (https://en.wikipedia.org/wiki/S/MIME).

Who are these encrypted emails aimed at ? 
The Sender of these emails must have used a specific certificate to encrypt these emails.

Find out who sent them - and ask them to tell you who they are meant for. (And which Certificate they used to encrypt, and where they got that from).

I'm *pretty* sure PRPC isn't able to decrypt incoming s/Mime emails: and (personally) I think this is probably just fine:

I'm not sure what the use-case would be to give the Email Listener an s/Mime key (or indeed a set of keys?) enabling it to decrypt incoming emails - only (presumably?) to translate them automatically into plain-text and save them ? (The *transmission* encryption - see below - is enough I would say?)

(Note: PRPC *is* able to *encrypt* (outgoing) emails for sure: but that doesn't require a secret key - only a public certificate: and there *is* a use-case for this).

The PDN articles given above, although they also relate to encryption : relate to the *transmission* of emails in a secure way - once the email have reached their destination, they are effectively plain-text again.

An analogy:

SSL/TLS here acts a 'secure envelope' while emails are in transit; but S/Mime means the 'letter' itself is written in a 'secret code'.

I hope this makes sense; and if I'm wrong about the Email Listener not being able to decrypt - I hope somebody can correct me.