<input type=hidden name='old_email' value='<?=$member[mb_email]?>'>
        <TR bgcolor="#FFFFFF">
            <TD class=m_title>E-mail</TD>
            <TD class='m_padding lh'>
                <input type='hidden' name='mb_email'>
                <INPUT class=m_text type=text name='mb_email1' size=10 maxlength=100 required itemname='E-mail 계정아이디'
                    onblur="document.fregisterform.mb_email.value=document.fregisterform.mb_email1.value+'@'+document.fregisterform.mb_email2.value">
                @
                <INPUT class=m_text type=text name='mb_email2' size=20 maxlength=100 required itemname='E-mail 도메인'
                    onblur="document.fregisterform.mb_email.value=document.fregisterform.mb_email1.value+'@'+document.fregisterform.mb_email2.value">
                <select name='email_domain_select'
                    onchange="if (this.value) document.fregisterform.mb_email2.value = this.value;"
                    onblur="document.fregisterform.mb_email.value=document.fregisterform.mb_email1.value+'@'+document.fregisterform.mb_email2.value">
                <option value=''>직접입력
                <option value='hanmail.net'>한메일
                <option value='naver.com'>네이버
                <option value='gmail.com'>지메일
                </select>
                 <a href="javascript:mb_email_check();"><img width="70" height="20" src="<?=$member_skin_path?>/img/join_check_btn.gif" border=0 align=absmiddle></a>
                <? if ($config[cf_use_email_certify]) { ?>
                    <? if ($w=='') { echo "<br>E-mail 로 발송된 내용을 확인한 후 인증하셔야 회원가입이 완료됩니다."; } ?>
                    <? if ($w=='u') { echo "<br>E-mail 주소를 변경하시면 다시 인증하셔야 합니다."; } ?>
                <? } ?>
            </TD>
        </TR>


이런 코드로 대체해 보았습니다.