ORIGINAL RECEIVER PLUGIN:
Use the Kopano Dagent plugin instead: https://git.sprinternet.at/kopano_core/kopano-dagent-rewritegaladdresses
The dagent plugin has a few benefits over this deprecated Webapp plugin: the changed receivers are also visible via IMAP and Z-Push.
Building the original receiver plugin from source:
Compiling the plugin
The plugin needs no compilation. Simply copy the folder to the webapp plugin folder.