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.
secret correspondence; confidential report
Emphasizes the confidential nature of the communication, often used in contexts like diplomacy, business, or military where secrecy is important.
This is a private letter, so please don't show it to others.
You should avoid sending personal messages from your company email address.
The diplomat sent a confidential report to his home country.