Mike uses POP3 to get his email on his Google Nexus 7 Android tablet, but he doesn't get all his unread email. Leo says that it could be another email client is downloading messages and taking them off the server.
If that isn't the issue, Leo says that using gmail to collect all the pop3 email and then accessing it via gmail on his Android tablet may be the best option. Leo also likes IMAP better, because it leaves all the mail on the server. The Mercury Mail app that Mike has also may have a messed up configuration.