Kaspersky is currently not compatible with Postbox, and could cause the system to freeze when the two are used together.
From Kaspersky Tier 2 Technical Support on 10/12/2016:
This is a known bug that the developers are working on. Currently the work around is to exit the program or temporarily disable the file AV to launch the program. A more permanent resolution will be released in a future patch/build.
Please follow the steps below to exit Kaspersky.
Please temporarily disable Kaspersky (right-click on the 'K' icon located near the time in the bottom right hand corner of your computer screen and select 'Exit').