Export your address book as a .CSV file. Excel can open .CSV files.

No cannot do this. All emails are not in my address book. I copied emails from different sources on to one email and then discovered I couldn't send it because it was too large.

Mailing Lists can get around the limit on the number of addresses. But you would have to add the addresses to your Address Book. I'm not clear if this would suit you, since you have already apparently decided not to do this.

Do you care about exposing the addresses to other recipients? Are you using Bcc:?

Mail Merge is another possibility.

https://addons.mozilla.org/en-US/thunderbird/addon/mail-merge/

I don't do this myself, but the once or twice I've tried it, I was uneasy at how well Mail Merge in Thunderbird handles errors.

The LibreOffice suite contains all you need for mail merge; a database or csv-file based address list, an smtp client for sending, all stitched together by its Writer word processor.

Due to time factor I had to do old fashion way, copy message, drop into new email and remove one email at a time from original email message and transfer to new email. Technology is great when it works but you know s--ks when it doesn't fit the need at hand. Thanks for trying.

To answer your question, so long as they are formatted properly, you can copy-and-paste email addresses from just about anywhere. I think I'd use a simple text file myself, but I can see that a spreadsheet might have useful additional capabilities such as the ability to search, filter and re-sort and also store supplementary data.

I found if I entered a number of addresses in a column in my spreadsheet (i'm trying this with the LibreOffice Calc spreadsheet program), copied that column and pasted into the address box in Thunderbird, they were entered correctly separated by commas. So far so good.

If you hit return after entering a comma-delimited list like this, Thunderbird will split it out into its preferred format of one address per line.

Note that when you send, unless you have disabled the feature, all the addresses not already in your address book will be added to the Collected Addresses address book. So they end up in your Address Book anyway.

If you do use Bcc: (please!) set it before you paste.

I use an add-on that places a delete button (it looks like an 'X') in each addressing box. This makes deleting email addresses easier.

https://addons.mozilla.org/en-GB/thunderbird/addon/address-close-button/?src=api