mail.you.com has one IP number , but the reverse is localhost. ns4.de, sunx.org, cabi.net, celox.nl, jcaa.com and at least 100 other hosts point to the same IP. you.com use this as a mailserver. you.com is a domain controlled by six nameservers. All of them are on different IP networks. Incoming mail for you.com is handled by one mailserver at you.com themselves.