To send a message in Emacs, you start by typing a command (C-x m) to select and initialize the ‘*mail*’ buffer. Then you edit the text and headers of the message in this buffer, and type another command (C-c C-s or C-c C-c) to send the message.
compose-mail).
compose-mail-other-window).
compose-mail-other-frame).
mail-send).
mail-send-and-exit).
The command C-x m (compose-mail) selects a buffer named
‘*mail*’ and initializes it with the skeleton of an outgoing
message. C-x 4 m (compose-mail-other-window) selects the
‘*mail*’ buffer in a different window, leaving the previous current
buffer visible. C-x 5 m (compose-mail-other-frame) creates
a new frame to select the ‘*mail*’ buffer.
Because the mail-composition buffer is an ordinary Emacs buffer, you can switch to other buffers while in the middle of composing mail, and switch back later (or never). If you use the C-x m command again when you have been composing another message but have not sent it, you are asked to confirm before the old message is erased. If you answer n, the ‘*mail*’ buffer remains selected with its old contents, so you can finish the old message and send it. C-u C-x m is another way to do this. Sending the message marks the ‘*mail*’ buffer “unmodified,” which avoids the need for confirmation when C-x m is next used.
If you are composing a message in the ‘*mail*’ buffer and want to send another message before finishing the first, rename the ‘*mail*’ buffer using M-x rename-uniquely (see Misc Buffer). Then you can use C-x m or its variants described above to make a new ‘*mail*’ buffer. Once you've done that, you can work with each mail buffer independently.
The variable mail-default-directory controls the default
directory for mail buffers, and also says where to put their auto-save
files.