After an update a couple of months ago I am losing emails from my Z30 (OS10.3.1.2582).

I use a Gmail pop account and have not modified settings for that. Received emails may remain for about a day, or often only a few hours and it doesn't seem to matter whether I've opened them or not on the Z30. Similarly it doesn't seem to matter whether I open them or not on my Mac or iPad, they still disappear from the Z30. Seen similar comments re Passport and Yahoo account but people focussed on Yahoo as the problem. I don't think it's the Gmail that's at fault in this case.