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535 incorrect authentication data error normally occurs due to wrong permissions.

For fixing mail box permissions from WHM,

1)> Log on to WHM as root. Then click on “Email” section.

2)> Choose “Repair Mailbox permissions”

3)> It will display the below message,

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

Click on “Proceed ”

This will fix permission issues in mail box

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