Bulk Delete Problems

Hi, I am using Mailbox in the Firefox browser. I want to find and bulk delete emails containing specific keywords, such as "booking.com". But when I search for specific keywords multiple weird things happen; For instance, I know that I have many hundred emails containg the "booking.com" keyword, but if i select only the initially loaded ones (let us say 35 emails) and move them to the trash bin, then searching for "booking.com" in the inbox again no longer returns any results. Furthermore, the number of selected and "deleted" emails in/from the inbox often do not macth the number of the emails in the trash bin. This all in all makes it quite frustrating to clean up my inbox. How to solve this problem? /Tue