我们公司自己搭的Exchange邮件系统,昨天发现有个邮件一直在队列里发不出去,上面提示上一错误为:451 4.4.0 Primary target IP address responded with:"421 gmx.net (mxgmxus001) Nemesis ESMTP Service not available." Attempted failover to alternate host ,but that did not succeed. Either there are no alternate hosts, or delivety failed to all alternate hosts. 请大神指导下!