You will not be able to edit the MX record for your domain unless you are using the 1&1 name servers. If you are using a CNAME record or other DNS, you will not be able to make a change to the MX records within the 1&1 Control Panel and will need to make this change with your current DNS host.
Please note that mail server priority is usually set in multiples of 10 with the lowest number taking top priority (kind of like golf). You must enter at least two mail server addresses for the MX record.
A confirmation page is displayed informing you that the changes will be updated accordingly. As with all DNS changes, it may take up to one hour before the changes are recognized by all servers/computers on the Internet.