The Bounce icon has disappeared from my mail bar.

Does Mail open normally?

You can try resetting the Dock.

Try doing a Dock restart using the following Terminal command:

killall Dock

Log out/in and test. If that doesn’t work, you need to look in your user Library/Applications Support/Dock for the .db. Use the Finder “Go To Folder” command. Enter ~/ Library/Applications Support/Dock. Move the .db to your desktop.

Then try a dock reset.

Applications/Utilities/Terminal enter the command

killall Dock

Log out/in test. If it works okay, delete the .db from the desktop.

If the Dock is the same, return the .db to where you got them from, overwriting the newer ones.

That command was removed years ago. It should never have been there in the first place.

Most spam has forged 'From' addresses so all you would be doing is sending yet more spam to totally innocent people (who are probably already getting a lot of bounces because of this) and making a bad situation worse. Even if the 'From' address is genuine, I very much doubt any spammer is going to remove your address from a huge list (they certainly would get a lot of bounces from actually dead addresses) - and in any case they can probably tell it's not a genuine bounce easily enough, so now you've just confirmed that your address works. This is why it was removed - even if you find a way to simulate a bounce, please don't.