Delay sending a mail, not possible ?
Pressing the send button on Mail App, the mail will first appear in OUTBOX a few seconds
Can I delete it quickly and stop the mail from being sent ?
Yes, you can delete it (or move it) but keep in mind that the computer is in the process of sending the message so when it will actually be sent varies. In my experience it is sent rather quickly so you have to move fast in order to remove/move it from the Outbox. It has worked for me occasionally but most times it's sent before I can stop it.
The other day I tried to quickly move a message out of the Outbox to the Drafts folder. It was moved but then I heard the Send confirmation sound. The message was not placed in the Sent folder but it was indeed sent, which I found out by checking with the recipient.
This experience over time has caused me to more carefully review the message content and to be sure it's something I definitely want to send.