According to a post on the Iranian Students Solidarity (Farsi) blog, sources indicate the information comes from a group of resisters who have infiltrated the administration and are leaking out important information.
Translation of the text of the Iranian student activists source;
On 27th of Shahrivar (Day of Qods) when Iranians demonstrated again on the streets, the Iranian authorities in addition to blocking many internet sites, all over Iran, blocked or severely limited access to Yahoo and Google. Google did not react and its problem was resolved with 48 hours, but Yahoo sent a representative to Iran’s telecommunications ministry, to resolve the issue.Source:
During the meeting with Iranian Internet authorities and telecommunications authorities, Yahoo representatives were asked to provide Iranian authorities with the names (data) on all Iranian Internet account holders in exchange for removing the block/filter on the Yahoo website.
The Yahoo representative subsequently expressed that currently there were more than 20 million email accounts and providing such a list will be a very time-consuming process. To which the IRGC (Islamic Republic Guardian Council) replied by asking the representative to provide email accounts of those individuals who have Yahoo accounts and are publishing blogs.
Apparently this made Yahoo’s task a bit easier and the Yahoo representative agreed to provide such a list within a matter of hours. Upon the receipt of such a list, which included approximately 200,000 emails, by the Iranian authorities, the regime immediately unblocked access to the Yahoo.com website. The list went back as far as five years and included active and inactive accounts and blogs.
It is necessary to mention here that the Iranian Yahoo is managed by Yahoo Corporation in Malaysia (Kuala Lumpur).