Tried following your instructions to switch back to classic mail, as another respondant said: your instruction do NOT work.
Drop this new email version. It is sluggish, cumbersome, and many of the features from the Classic Mail don't work with the new email. A,so: under Settings there is no category to return to Classic Mail. Very disappointed in your lack of testing of the new email and for your poor customer service.

Hi there,
This help page explains how to switch back:
help.yahoo.com/kb/switch-back-previous-vers..
Please be sure you click on the settings wheel in Mail, not in Help. We have found that some users are trying to make the change to switchback to classic Mail in help.yahoo.com rather than in Mail.yahoo.com. You will not find the option to switchback in Help. You will find it in Mail.
Thank you!
Mail Support
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Roderick Dorr commented
I agree; no link to switch back to classic mail.
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Steve Peterson commented
cannot get back to the classic mail - HELP