mx00.1and1.com has no CNAME records.
Question and response
Question
dig mx00.1and1.com. NS
Answer
The response has no answer records.
Authority
1and1.com. 198 SOA ns-1and1.ui-dns.com. dnsadmin.1und1.de. 2013096354 10800 3600 604800 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0