To send an email message from Emacs, type C-x m. This switches to a buffer named *unsent mail*, where you can edit the text and headers of the message. When done, type C-c C-s or C-c C-c to send it.
The mail buffer is an ordinary Emacs buffer, so you can switch to other buffers while composing the mail. If you want to send another mail before finishing the current one, type C-x m again to open a new mail buffer whose name has a different numeric suffix (see Misc Buffer). If you invoke the command with a prefix argument, C-u C-x m, Emacs switches back to the last mail buffer, and asks if you want to erase the message in that buffer; if you answer no, this lets you pick up editing the message where you left off.
The command C-x 4 m (
the same as C-x m, except it displays the mail buffer in a
different window. The command C-x 5 m
compose-mail-other-frame) does it in a new frame.
When you type C-c C-c or C-c C-s to send the mail, Emacs may ask you how it should deliver the mail—either directly via SMTP, or using some other method. See Mail Sending, for details.