ICS stock email client consumes too much traffic for POP3 accounts
This is not an issue of your email client. This is the way POP3 works. In Gingerbread's email client, you'd had set leave copy of email on server. So, your ICS email client is downloading all past emails from server. Unfortunately, POP3 doesn't allow selective downloads. Your mails are being fully downloaded (not just header) with attachments too. That's the reason of high traffic. You should now un-check leave copy on server (to make sure, it'd not download an email repeatedly after its removed from local cache). After full download, everything will become normal.

If you can, I'd recommend to use IMAP instead of POP3. IMAP syncs everything (e. g. email read status) with server-client. If you delete an email locally, It'd be deleted from server too (provided its in configuration). So, you can keep emails on multiple devices in sync. IMAP allows you to download email header only, download attachments on demand etc. So, it can save your data.