You may be aware of some issues between you and the sender. However, that person may no longer be around or has left, yet there are still unresolved issues.
Dreaming of receiving a letter indicates that you may know about a certain issue between you and the sender. Perhaps that person has passed away, and there are still unresolved issues or unexplained situations between you. If you do not understand the content of the letter seen in the dream, it may suggest that people have withheld a lot of information from you, leaving you in a state of deception. Hidden knowledge is easily accepted when the conditions for understanding are met.
Dreaming of sending a letter indicates that if you personally send a letter, it means you have important information to tell the recipient.