Review the message before sending - If this tool is used often and there are multiple drafts, it would be great if the system could display the message which you are about to send. At the moment the messages are sent in "blind" mode. For instance in Mailchimp you can always review the messages before sending them. Quite often you find there things to be fixed.
There’s already a test email option before sending your mail merge. https://support.yet-another-mail-merge.com/hc/en-us/articles/115000598389