By default it will place that copy in the "Sent" folder of the account it is sent from. But you can change this to be any folder in any account. For example, you could collect all sent mail from all accounts in the "Sent" folder of "Local Folders".
Specify Where Sent Mail Should be Kept in Mozilla Thunderbird or Netscape
To specify where copies of sent messages are kept in Netscape or Mozilla:
- Select Tools | Account Settings... from the menu.
- In Mozilla and Netscape, select Edit | Mail & Newsgroup Account Settings.
- Go to the Copies and Folders sub-category of the desired account.
- Make sure Place a copy in: is selected.
- Choose Other:.
- Select the folder where sent messages should be kept.