You could test the new version before forcing it on your users - then give the user the option to upgrade/downgrade as fits their needs.
There are so many things wrong with this new version - it would take a book to list them all. Please revert users back to Classic so you don't make enemies of all your users.

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Charles Seaverns commented
Yahoo/ Verizon,
You really screwed up this situation which has resulted in a mess.
NOW straighten it out. You have had over 5 months to do so! -
Dick Wilson commented
WOW, you guys really screwed the pooch this time.
My mail is a horrible mess.
All of the new mail has a blue dot and invisible message.
Many lines only show top or bottom half of the text
Really ???????
Don't you guys TEST anything before you DUMP IT ON US ?
Classic Please and go take a long vacation. -
Carol Swann commented
CLASSIC back PLEASE … this new version is a joke
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Rachel commented
The new mail is horrible and a huge downgrade from the previous version. It is super buggy and does not have the same features as the old one. I really really hate it. I have used Yahoo for years and it is being ruined and turned into a sleazy ad driven shlock platform
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Vincent Marino, Jr. commented
I want to go back to Classic Mail or leave Yahoo totally. I tried this new version and it stinks. How do I get Classic Mail back.
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Steve Tori commented
Your e-mail sucks
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S MACK commented
I don't like the BS new mail...I want the Classic because it works for me!
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Gene Schneider commented
I hate the new mailbox format. Only 12 characters visible in the Sender column, and only 39 characters visible in the Subject column. The entire table is squished into only 6" on the screen, no matter how wide the screen is.
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Brenda Wolfe commented
amen!
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Anonymous commented
obviously YAHOO could give a flying rip about what we want.