We keep emails forever as long as they're not deleted. It's highly possible that if your old mail client was using POP that it's "Leave a copy of messages on the server" or equivalent setting was disabled, in which case the POP client would have downloaded and then deleted the messages from Sonic. If that were the case, then we would no longer have a copy.
We keep snapshots of email folders so in the event of accidental deletion they can be recovered as long as it's within 7 days or so (but there are factors that could affect this, so it's best to contact us for recovery immediately to prevent permanent loss).
The definitive way to find out if we still have them is to login to your account at
https://webmail.sonic.net (webmail is just an IMAP client) and see what messages are there. Navigating to the last page of messages will show you the oldest ones we have, and the search bar near the top right of the message window is helpful if you know subject, sender, or content of what you're looking for.
IMAP clients leave messages on the server until they're deleted and then removed from the trash.
Hope that helps, post back if you have any questions.
