Move to trash
secure empty trash (finder menu)
In finder preferences (advanced tab) you can also tick the box "empty trash securely" to make this the default behaviour
For the Time Machine backup: use spotlight to search on your Time Machine disk for this file, also move it to the trash and securely empty it.
the .trash folder is hidden in your user folder
maybe this link is useful to you:
How is it possible to move a mail message directly from Mail to the Finder Trash, where there is the Secure Empty option? It is not possible to drag a message from Mail to the Finder Trash.
I would think this has to be done by first moving the message to the Mail Trash by deleting it, then navigating to the Mail folder in the user Library>Mailboxes>Deleted Messages.mbox and finding the specific e.mlx message there. Then move that to the Trash and use Secure Emtpy.
I don't use TM, but I know there are caveats for deleting using the Finder. This can be risky.
Thomas A Reed had this to say regarding deleting email from TM.
If you deleted files from your Time Machine backup using the Finder, then you've just destroyed your backup, most likely. You're probably going to need to reformat the hard drive, unless you are able to put all the files back exactly where you moved them from. Time Machine backups have some special quirks and are not meant to be accessed via the Finder.