Can't change contact names.
Can't change contact names in New Mail. Can be changed in old mail, but the change does not come across to new.
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If you still find that the issue is happening, please see this help page for troubleshooting:
https://help.yahoo.com/kb/new-mail/fix-problems-mail-sln28075.html?impressions=true
If it occurs after that, let us know in our forums:
Thank you!
Mail Support