IPhone – iPad mini virus? iOS 10.1 what to do

A spammer could validate reception by inserting an image which is hosted on a remote server in a HTML email. A unique code per recipient could be appended to the URL of the image, at which point the remote server can know which addresses recieved and opened the email.

While the default setting on iOS is to load remote images automatically (and hence makes your email address vulnerable to this kind of tracking), you can change this to Off in Settings - Mail, Contacts, Calendars - Load Remote Images.

Please note that this will prevent any remote images in any email to be loaded automatically, and therefor possibly breaking the layout of HTML emails, but you can always manually load the images in an email by tapping the appropriate button at the bottom of the email body.

There are no steps you can take because you don't have access to the server logs at Yahoo! If this is a legal matter, you might be able to obtain a search warrant for the mail server logs if law enforcement is involved. This would only prove where the connection was established from, which may or may not be useful.

In the meantime, have the owner of the Yahoo! account change their password to something stronger and ensure they don't leave their iPad or any other device logged in where somebody else can access it.