I received a new stamp from my friend, but I can't use it in my letter. Why is that?

Just like in the real world, you cannot reuse a stamp that you have received from your friends. When you receive a stamp from a friend in Slowly, it will be added to your Stamp Collection, which is like a stamp album for collection and viewing purposes only.

Only stamps in your "My Stamps" list are usable in your Slowly letters. Stamps could be unlocked when you meet certain requirements within the app or purchase from the Stamp Store.

Read more:

What are "Stamps" in Slowly, and how can I obtain them?