Mail Merge is a process of merging or combining two or more documents or data sources into a single document, primarily used for mass mailing purposes. The opposite of mail merge is "unmerge," which means to separate the merged documents or data sources into individual files or components. Another antonym for mail merge is "individualize," which implies personalizing the content for each recipient or customer, rather than sending out generic, mass mailings. Other antonyms for mail merge could be "separate," "distinct," "distinguish," or "divide." Each of these words implies the opposite of the process, which is to combine or merge separate components into a single document.