Problem: When "Membership: Anyone can register" is enabled (for multi-user community blogs), spammers are creating hundreds of new accounts using temporary email address services. So they'll receive the "Your username and password" email confirmation; giving them access to post full entries on the community-blog website.
Proposed Solution: Provide an easy way for admins to maintain a block list of domains that WordPress cannot send emails to, pre-populating the list with the most common temporary email address services.
My Experience: While I do receive an email notification when a user creates a new account, I have to manually delete those users who are obviously using temporary account names and email addresses. While the free @hotmail.com and @gmail.com accounts are less obvious, I have a list of 20 temporary email domains that are frequently used.