There are essentially 3 ways you can Mail Merge with email.
Ad 1. This will open an HTML (or text) editor which lets you insert mail merge fields. The resulting template is store inside the [YourDatabase]_processes.xml file
Ad 2. This requires that you checked ‘Document’ on the process tab. The check results in the appearance of an extra tab which lets you create the template.
Ad 3. Useful if you just want information which is concatenated in an expression on your form or query field.
Note: Using 2. [Use mail merge document] offers the best formatting and supports tables in your email. However as it is Word based it is significantly slower than using 1. <HTML> (or <Plain text>).