@Adam87
I use the list of reservations to find a guest. It has a least a fliter on "time-period".
(but would be nice the filter could also act on guestname ! )
Or download the csv file and find the guest + time period.
Then (from the specific reservation) click from the pulldown menu at the end (the three dots) to the belonging message box..
Just an "alternative search option" as long there is no direct search on the Inbox.
BTW
As all communications go also to my private external emailbox, it is even more easy to search a guest there, using the search/filter tools of the email-software.