Both the build-in mail client (Android 2.1) and K-9 mail, display the most recent
n messages received during the last
m days, where
n are configuration items. Messages older than
m days are invisible.
It's certainly sensible to hide old mail my by default, but how cannot I lookup an old message in INBOX or another folder? Even if the INBOX folder is huge, retrieving all headers should be an option.