mx0.gmx.de has one IP number , but the reverse is mx0.gmx.net. mx0.gmx.net point to the same IP. hvdb.de, pylos.de, fjs.info, aat-ct.de, brekky.de and at least 100 other hosts use this as a mailserver. gmx.de is a domain controlled by two nameservers on different IP networks. Incoming mail for gmx.de is handled by two mailservers at gmx.net. They are on the same IP network. gmx.de has one IP number. mx0.gmx.de is hosted on a server in Germany.