If I understand your question, then I imagine that adding all your 200 messages as attachments would do the job.

You can add any email message as an attachment by drag and drop onto the new message; drop onto the addressing box, not the message body window.

So your next challenge is to select all the wanted messages. You may be able to sort, or filter, and do a multiple selection. You could copy them to temporary folder, then do a select all. Having selected, you can drag all in one lump.

The cumulative size of all these messages, particularly with attachments could be quite large and may fall foul of mail server limits, so zipping them may be of benefit. So if you copy them to a temporary folder in Thunderbird, you can export that folder to a regular folder on your file system, and then zip those exported files, then attach the zip to your new message.

For bulk export, I'd use the ImportExportTools add-on. And use the .eml format for your export as it retain all the message content, and attachments.

An alternative approach might be to re - email them using the Mail Redirect addon. This sends the selected messages to a new recipient, but retains the appearance of them having come from the original senders. This means they will arrive in the recipient's mailbox just like any regular email and will be easier to access. The attachments procedure described in my previous posting will require a little more work to actually see the attached messages, and of course a zipped file is probably the least convenient, particularly for your recipient.

If it's important that the messages be unchanged, maybe as evidence for a lawyer, then the zip method is much the best approach.

I need to send an attachment to 50 bcc: email addresses. How do you send to a group and not to 50 different individuals then having to change every address from TO: to BCC ???

If you're going to be sending to this same group again, consider building a Mailing List. In the Address Book, File|New |Mailing List.

If it's a one-off, then you'll be adding 50 discrete addresses. How do you like to do this?

I would usually add addresses via auto complete. I start typing an address, the address book offers matching addresses, I select the correct one, hit return, and a new box opens for the next address. The trick is to set the first one to Bcc, before hitting return, then any subsequent boxes will adopt the current mode, Bcc in this case.

If you use the Contacts Sidebar in the Write window, there's a button labelled Add to Bcc.

You can also drag and drop selected addresses into the addressing box. Just take care to set the box to Bcc first. You could also paste in a suitably formatted string of multiple addresses (comma separated), again making sure the box is set to Bcc first.

And you can find an addon that lets you set all the currently set-up addresses en masse to one of To/Cc/Bcc. I think it's this one:

https://freeshell.de/~kaosmos/morecols-en.html

However, Thunderbird won't let you add as many as 50 separate addresses. So you'll need to

1. do it in several smaller batches, or
2. use a Mailing List, or
3. use a Mail Merge addon to create 50 separate, uniquely addressed copies of the message, or
4. use something like an office suite and its built in mail merge capabilities. Libre Office can do this for you.