Rodex Question

[GodKnows Jhomz]

Anyone knows what is the use of the encircled in the image?

I have unread and already read mail and it stores in the 1st Mailbox.
Can anyone please explain me the use of that other mailbox which is in gray scale.
Cause when you delete a mail, it will be deleted permanently so I think its not like a trash mailbox.
Thanks in advance.

[Akkarin]

Iirc that tab is for messages that are returned to the character after x days of not being opened by the original receiver.