Please check the Mac app with an antivirus before launch as it is downloaded from the developer's website, and we cannot ensure that it is safe. HTMLMailer for Mac lies within Communication Tools, more precisely E-mail.
From the developer:
HTMLMailer integrates with Address Book, so that you can send HTML email to a group of friends or colleagues as you wish.
You may want to check out more software for Mac, such as Zimbra Toaster, PageSender or Mail to ToDo X, which might be related to HTMLMailer.