535 Incorrect Authentication Data

The error normally caused due to incorrect permissions. We can fix mail box permissions from WHM on cPanel servers. For that : 

1) Log into WHM as root and click on "Email" section.
2) Select "Repair Mailbox permissions"
3) It will show a message like :

This function will inspect mailbox ownership and permissions and attempt to 
fix any issues that may exist. 

Click "Proceed >>" button.