private letter; personal message
Refers to a letter or message intended for a specific individual, not for public or official use. Can be used for both physical letters and digital messages.
This is a private letter, so please don't show it to others.
secret correspondence; confidential report
Emphasizes the confidential nature of the communication, often used in contexts like diplomacy, business, or military where secrecy is important.
You should avoid sending personal messages from your company email address.
The diplomat sent a confidential report to his home country.