Thunderbird itself does not use AI. Moreover, Thunderbird is a local email application, meaning it installs and runs as a client on your device. It does not provide email addresses, nor the service to transmit emails. But Thunderbird can be used with your existing email address and service provider to send, receive, sort and search your email messages. Wrt your concern about Gmail and AI, this is completely unrelated to the email client you use to access your Gmail account. It is of course up to you not to use Gmail as your email provider if you feel uncomfortable about what they are doing. For standard, free users, Google maintains that they do not train Gemini or other large language models on your email content. Features like Smart Compose or Automatic Tab Sorting do "read" your emails to work, but Google clarified that these are localized processes. Turning them on does not grant Google the right to use your emails for AI training. If you explicitly use Gemini Extensions (where you ask the AI to "find the receipt in my Gmail"), the specific prompts you type and the resulting interactions may be used to improve the service, but the rest of your inbox remains private. Note, every email provider is scanning your messages for malware, a.k.a. they are all 'reading' your email anyway.