That's bad. I'm not sure what happened there.

My guess is that SimpleMail created a POP3 connection to the server, downloaded the last six months of mail, and then deleted it from the server. That's a bad default setting if it did that. Your mobile phone probably was set up with an IMAP connection, where it synchronizes with the server constantly rather than downloading and clearing. This article has more information on POP vs. IMAP, although, it comes a bit late for you: Glossary of Terms including types of Accounts.

If that speculation is correct, we need to figure out a way to get the local copy of the messages back onto the server. (When you uninstall an extension, usually its accumulated data remains stored on your computer somewhere as long as you do NOT use Windows' System Restore or take other drastic action to "clean" the system.)

The add-on-author has a support forum here, although it doesn't look very active: http://www.userlogos.org/forums/support/simple-mail Could you try contacting the author there?

Hopefully that can be recovered quickly. Either way, new mail you send/receive should not be affected. What is the problem with sending/receiving new messages?

I tested Simple Mail in a new profile and it had a default setting of not deleting the mail from the server unless (A) you delete the mail from Simple Mail and (B) confirm that you want to delete it from the server. If you did that, I don't know if the deletion is recoverable. It would depend on whether the Simple Mail add-on put the messages in a Deleted Items or Trash folder on your computer, or completely trashed them with no backup.