''I recently got a new PC'' What OS Windows, MAC, Linux ? ''and was able to download Thunderbird '' Did you get Thunderbird download from the official website: https://www.thunderbird.net/en-US/thunderbird/all/ Or did you get it from 'Windows App Store' or 'Snap' or 'Flatpak' ? What version of Thunderbird did you install? ''and all my old received emails to the new PC. '' Did you create a new mail account OR try to import/copy old computer Thunderbird profile folder ? ''None of my old "sent" emails that I kept on my old PC downloaded'' What OS was the old computer? Did you have a Pop or Imap mail account ? They would only display if you created a new Imap account and those emails were stored on the server OR you manually copied original 'Thunderbird' folder containing all your profiles and data etc. ''and I am not able to send emails using Thunderbird.'' What is the error message - upload an image ? What server are you trying to connect? Post this info: * In Thunderbird * Help > Troubleshooting Information * Scroll down to 'Mail and News Accounts' section * Copy all data and post int his forum question OR upload an image showing the info.