Differences between the Actor Model and Communicating Sequential Processes (CSP)
Here is how I think Erlang works. I believe Akka is very similar.

Each process has a single mailbox. Messages are put into the
receiver's mailbox by the sender, and fetched by the receiver using
pattern matching. This matching process can change message ordering in
the sense that the oldest message in a mailbox may not match, but a
younger one does. In this case the younger one is consumed
first. Other than that, message ordering is preserved.

With this in mind, the asynchronous $\pi$-calculus extended with input
pattern matching input from buffer describes Erlang (and hence Akka)
semantics accurately, although one needs to do a bit of work in the
encoding, since the $\pi$-calculus doesn't have the restriction to
single channels per process. However, one usually doesn't want an
encoding, but rather a calculus that models the target system
directly.  Such a calculus exists, and it's called Featherweight
Erlang. It is by Mostrous and Vasconcelos. Their paper focusses on
typing, but you can ignore that and just look at the untyped calculus in Section 3.