What is the ruling on voluntarily painting someone else's property before asking the owner's permission?

Summary

Volunteer service such as painting mailboxes is a rewarded act of continuous charity. However, a person may not use another's property without the owner's permission, and asking permission in such cases is obligatory to protect neighbours' rights and good conduct. If acting without permission causes damage or loss, the volunteer must bear the loss. In the viral mailbox case, the painting was seen as causing no loss since it only replaced rust with grey paint, so forgiving the matter is encouraged.

Written in our own words from the original fatwa — check the source before acting on it.