I've used:
mail -s"Server $HOSTNAME booting" monitor@blah.com admin < /tmp/msg.tmp
to send an email to "monitor" and "admin" where the email body is the contents of the file msg.tmp.
Interactively you can do a:
mail -sSUBJECT who@there.org <ENTER>
then type any message content then when done type a single .<ENTER> on an empty line (I thin kit asks for a CC address) then off goes the email.
Check out "mutt" rather than "mail" if you wish to send an attachment.