There is no such thing as a Server address for thunderbird. Thunderbird is just a program running on your computer. You would have a mail account in Thunderbird set up as either pop or imap and it would currently contain the emails or perhaps you have those emails in 'Local Folders' mail account. If you have not so already, you would need to create a new mail account for the existing gmail email address and set up as an imap mail account (Thunderbird will auto configure as imap). first make sure you have cookies enabled. In versions 78* or later: * Select 'Menu app icon' > 'Options/Preferences' * In 'Options/Preferences' left Pane select Privacy & Security * Select checkbox 'Accept cookies from sites' * then File > New > Existing Mail account * Enter name, email address and password (the one you use to access gmail webmail account) * select checkbox to remember passwords * click on 'Continue' Thunderbird looks up configuration settings. User name is full email address Authentication Method will be et as OAuth2 click on create account Gmail will then prompt you to logon to prove you really are you and to ask to allow Thunderbird access. Check all account Settings are OK. In particular check the 'Synchronisation & Storage' settings. * https://support.mozilla.org/en-US/kb/imap-synchronization#w_configuring-synchronization-and-disk-space-usage Create suitable folders in gmail imap account ready to receive emails and make sure they are 'Subscribed'. Then select/highlight small batches of emails, right click on highlighted emails and select 'Copy' then paste them into the imap folder. When folder synchronises with server, the emails will get uploaded. This may take you a while to do. If you have an Anti-Virus product scanning all outgoing files then it can slow the process. Suggestion: you restart the computer in 'Safe Mode with Networking' mode and then start up Thunderbird. It may be a bit quicker to upload if nothing is scanning the files first. Your internet upload speed may play a part in a slowdown if uploading larger emails with attachments. Please note, there may be receiving limits imposed by gmail which may appear to slow down process. * https://support.google.com/a/answer/1366776?hl=en * https://support.google.com/mail/answer/22839?hl=en-GB Basically, you need patience and keep tabs on not exceeding limits.

Thank you for the comprehensive response.