Hi, I don't know about JRE, but regarding mail program try SimpleMail or YAM (both pre-installed in AmiKit).
Or simply use Thunderbird via RabbitHole feature. To create a new RabbitHole app in AmiKit, simply clone any existing one (including .info file), let's say Chrome or Safari located in RabbitHole:
Then text edit your new file and change Chrome (or Safari) to Thunderbird.
If you use AmiKit on Mac, right-click the AmiKit X app, select show package contents and go to Contents > MacOS > AmiKit > drive_c > Windows folder and do the same thing there - clone eg. safari.exe, edit your new thunderbird.exe file and change "safari" text to "thunderbird" (without quotes).
Make sure Thunderbird is installed on your Windows or Mac system.