Also, legitimate mail might not get delivered if the retry comes from a different IP address than the original attempt. When the source of an email is a server farm or goes out through some other kind of relay service, it is likely that a server other than the original one will make the next attempt. For network fault tolerance, their IPs can belong to completely unrelated address blocks, thereby defying the simple technique of identifying the most significant part of the address. Since the IP addresses will be different, the recipient's server will fail to recognize that a series of attempts are related, and refuse each of them in turn. This can continue until the message ages out of the queue if the number of servers is large enough. This problem can partially be bypassed by proactively identifying as exceptions such server farms. Likewise, exception have to be configured for multihomed hosts and hosts using DHCP. In the extreme case, a sender could (legitimately) use a different IPv6 address for each outbound SMTP connection.