'Collected Addresses' address book is usually setup as the default address book for collecting all email addresses you send when the email address/contact is not in Personal Address Book.

This is an option setup here: Menu icon > Options > Options > Composition > 'Addressing' tab select checkbox 'Automatically add outgoing emails to my: 'Collected addresses' Image below shows where it is, but in my case that checkbox is not selected.

You could use your backup (assuming you have one) and copy the 'history.mab' file from backup and paste into the 'profile name' folder overwriting the new default one. Thunderbird must closed whilst doing this. Second image shows where the 'history.mab' file is located.

Some addons that may be of use. https://freeshell.de/~kaosmos/index-en.html#addrcollector https://addons.thunderbird.net/en-US/thunderbird/addon/email-address-crawler/

The reply is greatly appreciated. Unfortunately, I have no backup mab file. The Email Crawler add-on looked promising; but when I tried to download it, I got the message that the file is corrupt. I do have the box checked, that tells Thunderbird to add email addresses to Collected Addresses when I send an email. But it won't add the addresses from old emails - only new ones. I may keep trying to download the add-on, somehow. I'm not very savvy about such things.

Upon reviewing the 2 add-on links provided, I found that Email Address Crawler could be successfully downloaded from https://freeshell.de/~kaosmos/index-en.html#addrcollector but not from https://addons.thunderbird.net/en-US/thunderbird/addon/email-address-crawler/ It is working perfectly!