Use up and down arrow keys for scrolling
While I like the new UI, its very annoying aspect is that up and down keyboard keys have been hijacked for "previous" and "next" message navigation. Those keys are normally used for scrolling long messages everywhere else. Without a mouse wheel, the only alternatives are crude PageUp/PageDown or trying to tap the narrow scrollbar.
Please assign some different keys for message navigation, such as Ctrl+Up/Down.

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Daniel Chai commented
Want to add to this - it's quite annoying that up/down aren't used to scroll through a page as virtually everywhere on the web. I didn't even realize how universal it was (and troublesome to not have it) until this change in Y! Mail.
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Kristin Knudson commented
Scrolling is very frustrating. If I place my cursor in the message and press the down arrow, It defaults to the next email message instead of scrolling through the current message. VERY frustrating.
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Chuck Dube commented
I use the down arrow to scroll through documents in other applications. It's a strong habit. Yet, it doesn't work when scrolling through a Yahoo email. This issue then turns subsequent e-mails as "Read". It's very, very frustrating. Please allow for down arrow functionality within an e-mail message. Thanks.
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Thanks for sharing your feedback with us! We will definitely share this with the rest of the team and see what we can do. Thanks again!