It's still in the program, but not in the toolbar. Apparently, it was deemed
not important enough or too infrequently used. You must got to the Message
menu and select it there. If you use it a lot, you may be able to give it a
keyboard shortcut using the System Preferences>Keyboard panel. An even
easier method is to set up Outgoing mail rules that select mail based on
address, subject, or category, and move the message after it is sent. If you
categorize your contacts (everyone in your organization goes in category
"work" and gets moved to the "work" folder, except your boss whose mail,
based on his email address goes to a special folder), it's easy to sort the
outgoing mail automatically.