I allways locate an "old" guest via list of reservations. This has a filter to select a certain period (I mostly remember the year the guest was staying). Search on the results can be done by browser search option. From the reservation it is possible to click to Inbox.
@Victoria46 I actually have a little book that I write down each guest's name, phone number, dates they stayed and their private email address, if they'd given me that (most do because I email them a map to my place and the coded email address that Airbnb assigns to guests seems to often fail as far as the map attachment being visible to the guest). Airbnb's software is so glitchy that I don't trust them not to somehow erase all my guest info at some point 🙂