Implement a mail merge in TiddlyWiki. Without writing JavaScript, you cannot actually send emails from TiddlyWiki, but you can produce a list of mailto: links that, when clicked, each open a new email in your email program with the email address, subject, and body for that recipient pre-filled. For low-volume tasks, this might be good enough.

On the tiddler that implements the merge, you should be able to fill in fields that provide a filter for contact tiddlers you want to send the message to, and some kind of template tiddler that uses a variable or a setting of the current tiddler to substitute in values that change.

You'll need to URL-encode the subject and body and put it into the link, which TiddlyWiki has a encodeuri[] filter operator to do. See https://en.wikipedia.org/wiki/Mailto for details of the mailto: URI structure; essentially, you want to produce links that look like this:

mailto:whoever@example.com?subject=TheSubject&body=TheBodyOfTheEmail

…where both TheSubject and TheBodyOfTheEmail are URL-encoded.

Hint: You will need the $wikify widget.