On July 15, 2013, Yahoo! is closing inactive e-mail accounts and opening them up to the public. That could spell trouble for you.
It would be bad enough if they were closing accounts, but they are actually going to re-open those accounts and allow anyone to secure those e-mail addresses. Besides the potential for bounced e-mails, which will affect your e-mail marketing analytics, this will increase your chances that you’ll be reported for sending e-mail spam.
Constant Contact explains the situation very well.
If Annette Smith loses her e-mail account and another Annette Smith comes along to secure it, then the second Annette will be receiving your e-mails. She might get annoyed by that, not knowing why she’s receiving them. If she reports you for spam, then you could end up getting a nasty letter from your ISP. The consequences could be dire.
This illustrates the perfect reason why you should clean up your e-mail list periodically. You should do it at least once a year – twice a year would be better.
You are better off deleting e-mails from your list if you can verify that the addresses to which you are sending them no longer exist than taking a chance of being reported for spam. Send out an e-mail to all of your Yahoo! account holders before July 15, 2013 and ask them to confirm their e-mail addresses. If you get no response, you should delete the e-mail from your list or set up a separate list for your Yahoo! account holders and run a series of tests.