Islam respects people’s private affairs to the extent that it declares spying to be prohibited. Therefore, reading people’s e-mail, letters, etc., without their prior knowledge is not acceptable in Islam.
Explaining the Shari`ah-based rulings regarding this issue, the eminent Muslim scholar, Sheikh `Abdel Khaliq Hasan Ash-Shareef, the renowned Da`iyah, states: “Reading someone’s e-mail without them knowing is some sort of spying and is not among the ethics and morals that a Muslim should be committed to. Islam forbids spying and makes it a grave sin. Allah says: “O ye who believe! Shun much suspicion; for lo! Some suspicion is a crime. And spy not, neither backbite one another. Would one of you love to eat the flesh of his dead brother? Ye abhor that (so abhor the other)! And keep your duty (to Allah). Lo! Allah is Relenting, Merciful.” (Al-Hujurat 12)
Also, the Prophet (peace and blessings be upon him) made following up the faults of people among the characteristics of hypocrites who believe with their tongues without their hearts. It is reported on the authority of Ibn `Umar (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) mounted the minbar (pulpit) and called people loudly: “O assemblage of those who submitted with their tongue without faith implanted in their heart, do not heart the Muslims nor follow up their faults. For the one who follows up faults of his Muslim brother, Allah will follow up his faults and will disgrace him even if in his house.”