A sealed letter is a variant of a letter that has a proof of authorship and integrity.
Seal impression and name of the sealer is shown in the item's tooltip.
Can be mailed.

Sealed Letter tooltip

Obtaining

Sealed Letters can be obtained by sealing a letter with a seal stamp.

Usage

Seal can be removed by holding the use button until a "seal ripping" animation is finished.
Removing the seal clears the authorship information and returns the item back to a regular letter.