To edit the MX Records you must supply at least two mail server addresses. The addresses must be domains or subdomains and cannot be IP addresses. If you are only able to access your mail server via its IP address, you must enter an IP address for your domain's MX Record instead.
Please note that mail server priority is usually set in multiples of 10 with the lowest number taking top priority (similar to golf). You must enter at least two mail server addresses for the MX record.