Mail Merge requires a Premium or Enterprise level plan.
Mail Merge is available using our legacy add-in within Outlook Exchange
When you send emails with Yesware, we rely on Gmail to insert your signature for you automatically. Since Mail Merges are sent not using Gmail's default interface, a signature is not automatically included in your outbound messages.
You will need to add a signature into your Mail Merge before sending the message if you want your messages to include a signature.