Don't roll all the messages into one
This new Yahoo mail STINKS. It's just like GMail, which I won't use for the same problem. I hate how email threads are truncated and I can't get back into them. I use Yahoo to plan weddings and I need to be able to see ALL the prior emails from my clients. I also hate that the emails are no longer sorted by date. Whose idea was this and who thought it was a good idea? On top of that, I can't even vote for all the other people who feel the same way. Yahoo, you BLEW it! Thank God my personal account hasn't been messed up with this. I don't want your new-fangled "improvements" in my mailbox! Leave me the heck alone.
You can disable the conversations feature by following the steps here: https://help.yahoo.com/kb/manage-conversations-yahoo-mail-sln28055.html