In mail.app, get list of recipients for just one of the many email accounts.
Here is a 2 step process to do that.

Part I

First create a Smart Folder in Mail.

There you will sort all emails specific to a selected account.



You can skip the above part and sort by account then select the messages from a single account.

Part II

Now use (Copy) one of those Apple Scripts (Thank you Matthieu Riegler) in to your AppleScript Editor.app

tell application "Mail"
    set theSenderList to {}
    set theMessages to the selected messages of message viewer 0
    repeat with aMessage in theMessages
        set end of theSenderList to (extract address from sender of aMessage)
    end repeat
    set AppleScript's text item delimiters to " "
    set the clipboard to (theSenderList as string)
    set AppleScript's text item delimiters to ""
    beep
end tell


Now back to the Mail Smart folder, and select as many emails as you want there (or all).

Back to Apple script, click run and it will extract the addresses of the selected emails to your clipboard. Once done....

Open new Notes or other empty text document and click Paste to get the addressees collected.