Email app not retrieving all emails

[natR]

I find that the Email app on both the G1 and the MyTouch does not retrieve all emails. Has anyone else experience this problem or does anyone have any debugging suggestions?

I have an email client on my computer set to leave all messages on the server. I never experienced similar problems with a Blackberry or other mobile email clients. K-9 seemed to experience this problem as well.

It's sort of the opposite problem of this post:
http://androidforums.com/android-applications/6177-way-block-read-emails.html