The introduction of IPv6 has made spam filtering more difficult.

Many ISPs have reported that the transition to IPv6 has increased the number of problems for security services that use blacklists of "bad" IP addresses to compile lists of spammers.

Spam filtering is a highly complex process, in which IP blacklisting plays a key role. Email service providers track the IP addresses of spam senders, block them, and create blacklists. If the address space is very large, spammers may use a single IP address to send an email and then change it to a new one, which can overload email systems.

To avoid this, some suggest setting a time limit on how long emails are stored in users' inboxes, while others believe that providers should not accept incoming email from IPv6 addresses by default, except for outgoing messages from their customers. Or they should avoid doing so altogether until they are sure the connection is coming from a reliable source.

It's also important to consider that IP address tracking is also used for other purposes, such as filtering. And an expanded address space will make life much more difficult for network defenders.