The IsEmailValid() routine tests whether the
specified field of the given form contains an email address of
valid syntax. As the actual email address cannot be truly
verified without some special magic, this routine shall at
least attempt to verify the syntax of the address.
If the given email address is not valid, an error is
displayed, the focus is set back to the specified form
element, and a value of FALSE is returned, otherwise a value
of TRUE is returned.