Polling Timeout on application edit page affects regular polling of basic info (databases, files, performance counters, etc.) which is being collected every 300 seconds by default. While data for "Server mailboxes" component are collected by separate job that is executed once per 24 hours and has also separate setting for job timeout (720 minutes by default). It can be changed globally (not per application) in config file "SolarWinds\Orion\APM\SolarWinds.APM.BlackBox.Exchg.Collector.dll.config" if needed:
Restart of Orion services is needed in order to apply change of this parameter.