The mailman in dreams is understood as the person who brings the most important news that must be known whatever you like it or not. If you didnt receive the news from the mailman, it could mean that something is hidden from you.
Dream dictionary: Mail carrier, mailman dream meanings
In a dream, a mailman represents the caller to God, a preacher, a news carrier, an emissary, a letter carrier or a courier. (Also see Dromedary rider | Mail | Ostrich)