An error you may receive is error: Record is too large when running a Simple Mail Merge through Export, if the export includes comment fields which are too long.
1. Open the Export and select File, Save As. 2. Save the Export as a different name and then open the new copy. 3. Select Tools, Export Options and change the Format from Simple Mail Merge to Excel. 4. Export to Excel and search for the long comment. 5. Shorten the length of the comment and re-merge the letter.