Mail mode has two commands for sending the message you have been editing:
C-c C-s (
mail-send) sends the message and marks the mail
buffer unmodified, but leaves that buffer selected so that you can
modify the message (perhaps with new recipients) and send it again.
C-c C-c (
mail-send-and-exit) sends and then deletes the
window or switches to another buffer. It puts the mail buffer at the
lowest priority for reselection by default, since you are finished with
using it. This is the usual way to send the message.
In a file-visiting buffer, sending the message does not clear the modified flag, because only saving the file should do that. As a result, you don't get a warning if you try to send the same message twice.
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