There are two problems in restricting access from Yahoo! and Hotmail
email addresses:
The first is that it would limit a lot of honest users. If we allow
anonymous contributions, marked only by IP address, we can surely allow
people with any email address to contribute.
Secondly, doing this would not hinder some people. For example, I can
create any number of email forwards in my domains in a few seconds.
True, you can block the entire domain, but there are some domains with
many legitimate users.
Besides, this isn't what Wikipedia is meant to be. Identity verification
is all well and good, but not when there are so many innocents involved.