If you are suffering from a lot of spam registrations on your blog, or even just people creating multiple accounts in order to troll, then you may find it helpful to block registrations coming from free email addresses.
This may cost you some real registrations from people who either do not have, or choose not to use, an ISP email address, or an email address from their own domain. It is up to each individual webmaster to decide if the possible lost registrations is made up for by the reduction in spam and fake registrations.
It is worth remembering that MSN and Yahoo email addresses are given out by some ISPs. People who have premium email accounts with one of those domains attached to them may feel annoyed when they see they cannot register on your site. For this reason, it’s worth explaining to new users why you have decided to ban certain email addresses, and perhaps offering an alternative signup method.
If you do decide to block free email addresses, then this list of free email domains is a very good starting point. This is an old list that was posted by Welo on the VBulletin.com forums. The domains have been checked to see which ones are still current, but new ones have not been added.
The list is a good starting point, and can be added to if you get new spam registrations from other domains. Combined with other spam fighting techniques – such as blocking bots in your .htaccess file, you should find that your WordPress-MU blogging community becomes a clean, spam free place for your users to hang out.