emails on my phone (accessing a pop account) stay in the inbox for 14 days then fall out. my wife loses hers after 7 days. These are not deleted items, just the mail box doesn't have emails older than those days back, even If we didn't delete them.
I saw a setting in iphone settings, there's a keep deleted items x days. but these aren't deleted. they are in the inbox and maybe read, maybe not.