To dream of an envelope represents a personal or private message. A signal that you or someone else has something important to say.
To dream of opening an envelope may reflect waking life situations where you or someone else is finding out about a personal message or private matter. Accepting an opportunity or choosing to learn the truth.
To dream of purposely leaving a letter unopened may reflect a lack of interest in communicating or being associated with someone. Not being interested in what someone else has to say. Feeling that an issue, secret, or opportunity is best left alone. Preferring to live with uncertainty. Negatively, it may be a sign that you are afraid of learning the truth or learning more about a sensitive issue.