IIRC, there are "Start Over" and "Cancel" options on the same screen as the "Send Message" button. Once you've actually hit "Send", though, the message is on its way.
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Cap'n Q
My first SE IV mod!
Hypermaze quadrant
The most merciful thing in the world, I think, is the inability of the
human mind to correlate all of its contents. We live on a placid
island of ignorance in the midst of black seas of infinity, and it was
not meant that we should go far. -- HP Lovecraft, "The Call of Cthulhu"