An envelope in a dream is an exciting and wonderful opportunity with a message inside. A message could be bad news, something important, or simply the chance to have some fun.
Dreaming about an envelope represents the importance of the content inside. The importance of this content might need to be made clear to you by somebody else who could get it for you. Your dreams can be put into context of your life by this person who can show you that what you want is possible. For example, if your family life is not going well and there are problems with money then this envelope could indicate that you should step up and take the next step.
Envisioning an envelope in a dream means private messages, the wish to communicate with someone and the desire to get back in touch with whatever it is you want to say.
A dream in which you receive an envelope may have a negative meaning also and it may refer to something that is about to happen or an event that will occur in the future. The contents of the letter will usually reflect more on your inner mind than on what is happening outside of your dreams.