Try each of the following steps in order until the issue is resolved:

  1. Run the mail merge for smaller groups of records:
    1. Create a query that includes half the records in the mail run.
       OR
       Use the filters on the tabs of the mailing (e.g. General and Filters) to run the mailing for a smaller group of records.
    2. Run the mailing using the query or the selected filters.
    3. Repeat steps 1 and 2 for each remaining group of records.
  2. Recreate the mailing.