Login yahoo mail and choose you email like inbox email, sent email, trash email, spam email..etc. Then open related email and you see email left side box icon making just select and delete or just email top more option like make simple box image, delete, move, spam..etc. So just box image click and select which email delete or if you are all email delete then click select all email one by one, or go to delete option and click it.
If not understand contact yhaoo email technician.
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