Move to the beginning
of the body of the message
(message-goto-body).
C-c C-i
Move to the
signature of the message
(message-goto-signature).
C-a
If at
beginning of header value, go to beginning of line, else go to
beginning of header value. (The header value comes after the
header name and the colon.) This behavior can be disabled by
toggling the variable
message-beginning-of-line.