A mailto URL specifies an email message to be sent to a given
email address. For example, ‘mailto:foo@bar.com’ specifies
sending a message to ‘foo@bar.com’. The “retrieval method”
for such URLs is to open a mail composition buffer in which the
appropriate content (e.g., the recipient address) has been filled in.
As defined in RFC 6068, a mailto URL can have the form
‘mailto:mailbox[?header=contents[&header=contents]]’
where an arbitrary number of headers can be added. If the
header is ‘body’, then contents is put in the message
body; otherwise, a header header field is created with
contents as its contents. Note that the url library does
not perform any checking of header or contents, so you
should check them before sending the message.
The value of this variable is the function called whenever url needs
to send mail. This should normally be left its default, which is the
standard mail-composition command compose-mail. See Sending
Mail in The GNU Emacs Manual.
If the document containing the mailto URL itself possessed a
known URL, Emacs automatically inserts an ‘X-Url-From’ header
field into the mail buffer, specifying that URL.