It also may be a function of what email server you are using. If gmail, there is a setting to only download new messarges, or starting now, I don't recall the exact phrase.
Also, I don't know what you have as email for Cacko, but IIRC, if you are using IMAP, it doesn't behave the same as POP mail. That may be another factor.
I like leaving email on the server so I can access it again if I need to later, even after erasing from my Z.
sdjf