I have my mail app set to retrieve 25 messages. When I delete a new message it reloads an old message (even if I recently delet ed it)... using my limited data.
Welcome to Android Central! Which email provider do you use? It sounds like you're using your provider's POP3 server, which is an old protocol, and can lead to this kind of behavior. Check with your email provider to see if they have an IMAP server, and use that instead. Read more here: IMAP vs POP3