Compose your Mail Merge email and then tap on the Review button to review your email and make other changes before sending.

You can scroll down the list of recipients and select them to preview the email with their unique merge fields and further personalize the message for each recipient.

There are several options when sending a Mail Merge email:

Tap on Review and select "Preview for a typical recipient".

Any Merge fields will also be automatically expanded. 

 

You can also expand your merge fields and keep editing for each individual recipient. 

 

Once expanded to reveal the content of the merge fields you can further personalize the message.