zimbraPrefGroupMailBy & zimbraPrefReadingPaneEnabled used to be options in Preferences > Mail tab. 7781 is implemented in ZCS 5.0.6+ which made them implicit (saved at end of session automatically) and removed them from preferences:
There's just a quirk in Zimbra Desktop for non-Zimbra accounts. (Doesn't save at prism quit & and as there's no server attributes to sync against so you might have to constantly turn off the reading-pane etc.)