Incoming mail automatically goes to trash. Why?
Some of my incoming email goes straight to trash and is marked as "read". When I move it to the inbox, it goes there temporarily and then returns to trash. What is going on?

When email goes directly to the Trash folder, it’s usually due to a filter set up on the account. Take a close look at your filters section to see if this is the case:
https://help.yahoo.com/kb/SLN15994.html
Also, syncing issues between yahoo and another program that accesses it can cause emails to be moved to the Trash. Check out this post for more info:
Thank you!
Mail Support
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Cynthia Thornton commented
Mine are doing the same thing! I've gotten to the point where I check my trash and spam first before doing anything and then I send what I want back to inbox and the rest I delete.
It's been extremely frustrating trying to apply for new jobs when all correspondence is going to trash!