Yahoo e-mail addresses hijacked?

Despite my best efforts at keeping my PCs squeaky clean, it appears my Yahoo e-mail contacts are being harvested and hijacked to send out spam.

The tipoff is that - every few weeks - I find a host of undeliverable failure notices (all at one time) for people I have not recently e-mailed. I'm thinking it is a problem at Yahoo's end; however I want to see if there is a chance something has infected my PCs that neither Malware Bytes or MSE is catching. Here are the details:

* I am only accessing mail from the Yahoo homepage. It is not downloading into any e-mail program such as Outlook.

* Neither Malware Bytes nor Microsoft Security Essentials detect any problems. Both are up to date.

* I have tried deleting my Yahoo e-mail contacts stored on the server. It appears whatever is causing the problem is able to harvest addresses directly from old e-mails stored on Yahoo's server.

* This happens on my work PC, home desktop and notebook. Again, no malware or viruses are detected on any of the PCs.

* I am religious about not clicking on e-mail links. If an e-mail appears suspicious I delete it immediately.

* This only happens on Yahoo. It does not happen on my Comcast or Microsoft e-mail accounts.

Any ideas on how to fix this? Thanks in advance for your help.