How do you know it "goes to Outlook"? I read that to say you're running Outlook. If you run Outlook, it will ask the server for messages. If it asks before Thunderbird does, it may gather all the messages for itself. Suggestions: # Stop using Outlook. Then nothing can "go to" it. # Configure Outlook to leave copies on the server. # Configure both Outlook and Thunderbird to use IMAP with your email servers. "Default" usually means the program that will be opened up when you do something connected with sending email. It doesn't affect which mail clients (programs) are started automatically, nor their acceptance of incoming email messages. So why would you want two email clients, both connecting to the same email accounts?