Delete Email on Server and HTC EVO

Every once in a while my phone will switch the check box "Delete mail
on server" from off to on. When it does happen, I lose all the new
emails on my server. It doesn't do this all the time, but it seems
that it occurs most often after I power down the phone.

Is this something uncommon or have others seen it? I'm on Android
version 2.3.5, software version 4.67.651.3.