Is it true that Allah does not forgive all sins—such as adultery with a neighbor's wife—even if the intention in repentance is sincere? And does this contradict the Quranic text: "Indeed, Allah forgives all sins except associating partners with Him"?

What is mentioned in the texts promising forgiveness for the sinner is specific to sins committed between the servant and his Lord. However, matters related to the rights of other people are not included in those texts. Imam An-Nawawi said: "Jihad, martyrdom, and other acts of righteousness do not expiate the rights of human beings; rather, they expiate the rights of Allah the Almighty." A man committing adultery with his neighbor's wife is among the gravest abominations and an aggression against the neighbor's right.