A user found an obscure bug in Outlook:
When you use Send To > Mail recipient or insert an attachment into a message before adding the address, and select a name from autocomplete that is over the attachment, you can send the message just by pressing S.
The autocomplete list was designed to be used with a single click and when you double click, it selects the address on the first click, adding it to the To field, then the second click selects the underlying attachment.
If you type S to add a recipient while the attachment is selected, you’ll send the message. Because the cursor is not in the address field, it won't bring up the autocomplete list of contacts whose name begins with S. The correct behavior should be "nothing happens".
There is definitely a bug here. Selecting an attachment and pressing S shouldn’t send the message, but you shouldn't be double clicking on the autocomplete address. It was designed for a single click.
If you don’t double click on the address, you won’t have a problem, although if you tab out of the To field and stop on the attachment, S will send the message too.
The workaround is easy: if you use a mouse to select entries from the autocomplete list, use a single click or select the address using the arrow keys and press Enter.
The keyboard shortcuts for send is Alt+S or Ctrl+Enter. In some instances, such as with meeting requests, Ctrl+S may send.