mx1.mailman.com has one IP number. mx1.69.net, mx1.um.net, mx1.ab.net, mx1.yl.com, mx1.a1.com and at least 100 other hosts point to the same IP. mailman.com use this as a mailserver. mailman.com is delegated to two nameservers, however both delegated nameservers are missing in the zone and two other nameservers are listed instead. Some of them are on the same IP network. Incoming mail for mailman.com is handled by one mailserver at mailman.com themselves. mailman.com has one IP number.