Hello there Roger Clarkson. '''''Can you provide us with more details?''''' Can you provide us with more details? Please list what antivirus you use, what VPN software you are using, and whether your SMTP settings are correct.This issue may be caused by outdated stored credentials, incorrect server settings, or the need to generate an app-specific password. '''''Correct Thunderbird Settings''''':> Server name: mail.btinternet.com> Port: 465 or 587 Connection security: SSL/TLS (for 465) or START-TLS (for 587) Authentication method: Normal password User name: Full email address (yourname@btinternet.com)''Misspelled entries often cause Thunderbird to reject them.'' '''''Asked details''''' : Security Check:Security software?OS: Mac or Windows?Are password combinations secure?Can they be changed ?do you use browser or desktop app?Do you use two step authentication in Btmail? '''''Reset''''' Try opening Settings (the three lines icon on Windows or 'Preferences' on Mac). Go to Privacy & Security and scroll down to the Passwords section.Remove old credentials: Click on 'Saved Passwords', then 'Show Passwords'. Find all entries for mail.btinternet.com (both incoming and outgoing) and click Remove.Restart: Close and reopen Thunderbird. It should now prompt you for your password. Type it in carefully.Important: Use the same password you use to log in to BT Webmail and ensure the username is your full email address. '''''Cache''''' Can you clear the browser cache? And all cookies?