Would it be possible to implement such feature in the mailbox. Would make it easier to delete all at once.

you can do it, click invert all i think

Agreed.. it works but it isn’t intuitive.

it is “invert selection” . i dont see a problem with that, but suggestions of more intuitive approuches would be appreciated.

We though invert selection would make things easier instead of having two options (select all / de-select all).

well i agree with the select all option but its indiferent to me