Ensuring accurate formatting for zip codes, especially when they begin with leading zeros, is crucial for the correctness of your mailing labels. In Excel, before performing your Mail Merge, format the cells containing zip codes as 'Text.' This preserves any leading zeros or specific formatting. To do this, right-click on the column header, choose 'Format Cells,' and then select 'Text' under the Number tab. When you proceed with your Mail Merge in Word, the zip codes will import exactly as they appear in Excel, maintaining the integrity of your mailing addresses. This step is particularly important for ensuring that your mail reaches its intended destination without delay.