Are you or someone you know using an older UI version of Yahoo! Mail or maybe even the Classic Edition? Then this news is going to be of specific interest to you. Starting June 3rd Yahoo! is pushing mandatory upgrades to the newest UI for all mail accounts and we have
the information you need if you are affected by the changes.

Some people will not have a problem with the changes while others are going to be very upset at
having to switch to the new UI. Either way, the upgrades are mandatory if you want to continue using your accounts as seen in the following excerpt. Take note that the upgrades also come with a new TOS/Privacy Policy update.

From the official announcement: Beginning the week of June 3, 2013, older versions of Yahoo! Mail (including Yahoo! Mail Classic) will no longer be available. After that, you can access your Yahoo! Mail only if you upgrade to the new version. You should have received an email from Yahoo! letting you know that your account required an upgrade.
When you upgrade, you will be accepting our Communications Terms of Service and Privacy Policy. This includes the acceptance of automated content scanning and analyzing of your communications content, which Yahoo! uses to deliver product features, relevant advertising, and abuse protection.

For those who may be looking at, and feel displeased with, the part that refers to “automated content scanning and analyzing of your communications content“, just remember that it is no different than using a Gmail account. Google does the same kind of scanning in order to deliver targeted advertising each time you read through your mail.
You can learn more about the mandatory upgrade via the blog posts linked below. We have also included reference links from Yahoo! for those who may have questions about (or wish to close) their accounts.