[dnanian]

Unfortunately, v3.2.5 is not Catalina compatible. So, you likely backed up the "Data" volume. On that will be your Mail folder, but it's going to take some work to find it.

Use Finder's Go To Folder command to open

/Volumes/the-superduper-backup-drive/Users/your-user/Library

substituting appropriately. Once there, you should see a folder named "Mail". Copy that to your Desktop.

Then, making sure Mail isn't running, you'll want to use Go To Folder to open

~/Library

When there, rename the Mail folder to, say, Mail.old. Then move the Mail folder on your Desktop here.

Make sure your networking is off. Open Mail.

Locate the email. Export it as explained above, to your desktop. Quit Mail.

Move the Mail folder back to your desktop. Rename the Mail.old back to Mail.

Start Mail. Import the mail you exported. Turn your network back on.