The antonym for "postmarking" is "receiving", which means to take delivery of something. When something is received, it means that an item or a document is being delivered, rather than being sent. If a letter or a package is being received, it is not being stamped or marked in any way, as the person who is receiving it is not involved in the process of mailing it. In contrast, "postmarking" refers to the process of stamping a letter or a package with a date and a location to indicate when and where it was mailed. Therefore, the antonym for "postmarking" is "receiving".