Deleting Older Email Messages iPhone

When accessing a POP account with an email client, all messages received at the incoming mail server that remain on the incoming mail server will be downloaded as new an unread messages with the email client and there is no way to prevent this. The only way to prevent this is by deleting the messages from the incoming mail server after being downloaded with the email client used on your computer to access the account.
With an IMAP account, all server stored mailboxes are kept synchronized with the server automatically with each email client used to access the account. Not possible with a POP account. The accounts Inbox mailbox with a POP account is not kept synchronized with the server. Any new messages that remain at the incoming mail server that have not been downloaded with the email client being used to access the account will be downloaded as new an unread messages even if the messages have already been read with another email client.
 MacBook Pro 15" Intel Core 2 Duo/2.53GHz/4GB RAM/500GB HD Mac OS X (10.6.6)  iPod w/Video 30GB,  iPhone 4 32GB