My dad has the same issues. Inbound spam is significant. Twice now his account has been hijacked to send spam outbound. Yahoo blamed it on him stating an easy password. Ironically his password is strong as its 12 characters, mix of upper lower case, with numbers and special characters. Researching this its apparent yahoo is having problems they aren't disclosing about unauthorized access to user accounts
Top on that all the add content that pops up when moving the mouse cursor around, and them being buddy buddy with the NSA. Failing to see why more aren't moving off yahoo mail and like others to services that provide web based similar service.
Nothing is free, reference above. An alternative might be your own domain with email service. Last time I renewed my plan with godaddy it's was like $65 for the domain and email service for two years including 10 email addresses
No ads, very little spam that's pretty easy to deal with and no email compromises for the 10 years I've been on the service. Plus I think godaddy is still a private company and not all cozy up with the NSA. They're not perfect but a reasonable alternative.