How can I create a subject line or body text filter that filters outgoing email?
When the filter matches a message, the message should not be sent.
How can I create a subject line or body text filter that filters outgoing email?
When the filter matches a message, the message should not be sent.
Filters are something that happen when an email is received, not sent - so what you ask for isn't possible with Gmail.
While not exactly what you are looking for, consider the Gmail Labs "delayed send" option. It gives you at the very least a short "undo" window.