This is an exclusive feature of Yahoo Mail account that you can recover your deleted emails up to some extents. This is helpful when you mistakenly deleted some emails from your account and later you realize the need to get back those emails in your account.
If you ever find that your deleted emails are actually important one and you want them back; simply check your Trash folder. This works in the temporary deletion; but if you deleted emails permanently then you need to make a request to Yahoo. This works in the first 7 days of your deletion.