If you've installed and setup Lightflow Lite, you will then need to use that app to manage your notifications, and I'm not sure Lightflow supports the Sony's email application. If you're using Gmail, for example, you can turn on the Gmail notification in Lightflow.
Have you tried setting a notification in Lightflow for something obvious like battery below 100% and seen whether that works. If not, you might have a different problem.
~~~~~~Sony Tablet S~~~~~
~~Rooted Android 3.2.1r2~~~
"The command-line: stopping noobs dead since...forever" (CS351, 2011)
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