Swetch, you're probably using gmail. Their email server acts a little differently from the others, and the account has to be configured in a particular way to avoid the problem you're seeing. Soon we'll release a gmail specific email reader that includes these instructions for configuring the account:
To ensure that you see your most recent Gmail messages, log in to your Gmail account and go to the Forwarding and POP/IMAP tab on the Settings page (http://mail.google.com/mail/#settings/fwdandpop).
In the POP Download section, select:
1. "Enable POP for mail that arrives from now on."
2. When messages are accessed with POP keep Gmail's copy in the Inbox.
Please try that and let us know if you start seeing the most recent 5 messages. Note that in the case of gmail it will currently also show outgoing messages if those are the most recent, as has been discussed in another tread.