Read Unread marks don't sync on cell phone
The read and unread marks don't exchange back and forth from PC to cellphone.. (Iphone 8).
2nd
ranked
Rob Base
shared this idea
If you are accessing your mail on the phone through the built-in app, make sure you have the IMAP settings not the POP ones: https://help.yahoo.com/kb/new-mail-for-desktop/sln4075.html