Mail exchange record

Provide a mail exchange record pointing to mx1.hdm-stuttgart.de. Test this configuration using dig accordingly.

Caveat: Configuring a client machine using your name server and sending a mail to xy123@g7.sdi.mi.hdm-stuttgart.de won't actually work since mail.hdm-stuttgart.de will reject mails being sent to any domain other than certain subdomain of hdm-stuttgart.de.