I have been usiong yahoo over twenty years till now, but I have to say his new edition email is really poorly designed.
When trying to replay to a sender, you would found it always replay to yourself. The email is not well arrayed, when click one of the earlier email and tried to replay to sender, you would often lost the mail. I understand the designer tried to compose everything in one folder but it seems that many things not so clear and not well designed. I would prefer the earlier edition. Hope you can change this soon.
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Have you tried general browser troubleshooting or a different browser? If not, please try these steps to fix this issue: help.yahoo.com/kb/sln2891.html