The following special commands are useful for sending mail:

^C	Sends the mail, after first asking for confirmation
^X^A	Aborts the mail, after first asking for confirmation
^Xa	Toggles whether or not your mail name abbreviations are expanded
^Xb or ^X^B  Toggles whether or not you'll keep a blind copy of the mail
	  (the mode line shows whether these things are currently planned)
^Xt	Puts you on the To: line with abbreviation mode on
^Xc	Puts you on the Cc: line with abbreviation mode on
^Xs	Puts you on the Subject: line with abbreviation mode off
^X> or ESC->  Puts you at the end of the file, with abbreviations off,
	and with the margins set properly for mail bodies
^X<	Puts you at the beginning of the body with abbreviation mode off
^Xf	Fetches a certain field from mail now in the bodies buffer
^Xi	Inserts the entire contents of the bodies buffer
ESC-j   Justifies (nicely formats) your current paragraph

See the BAGS manual for more details, including how to customize things.
The manual can be found in /usr/misc/.bags/doc/bags.{doc,ps}.