Hm, so single vars are working for you?
Maybe something's changed on Tasker side? I haven't touched this code in a couple of years probably.
The code puts "single" variables into return data like this:
bundle.putString("%aqmfrom", "lala@foobar.com")
and arrays like this:
final ArrayList<String> list = ...
bundle.putStringArrayList("%aqmfrom_array", list)
The dev guide
http://tasker.dinglisch.net/plugins.htmlstill says:
ArrayList (from v4.3) values are also supported
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If you enable Aqua Mail's debug logging (please see my signature below), then you should see output from Aqua Mail like this:
EventQueryReceiver varsBundle <a trace of variables that AquaMail returns to Tasker>