I think that's what happened to me a few months back, when I was having trouble with Thunderbird. I had to change the server type from POP to IMAP (had something to do with my ISP, I think).
I don't think you can actually make that change to an existing account in Thunderbird, though, so you'll have to create a new one. You know, go to Tools -> Account Settings -> Add Account -> put in your info as normal -> Choose IMAP and put "email.leamonde.net" under the incoming server field. I think.
Oh, and when you're done, go back into the Account Settings -> New Main Account You Just Created -> Server Settings -> under Security Settings, where it says "Use Secure Connection:" choose TLS, if it hasn't been chosen already.
There might be an easier way, though, I'm not sure. If there is, I'm sure someone else'll let you know.

(This is all assuming, of course, that you're currently using a POP mail server. If not, I have no idea what to do, lol)