Bulk Mac Mail is an an application for sending personalized mass email messages. It allows you to use message templates with multiple custom fields from the recipient database inside your messages. Customer lists can be imported from files of any type. Since all messages go through the standard Apple Mail's message delivery framework, they are absolutely indistinguishable from the messages that were actually sent by Apple Mail.
mailxel is a personal mail store and mail client with a focus on the management and organization of high volume mail boxes. The main features are fast search, tagging, GTD inbox management workflow, efficient data storage, address management, automated server-side mail folder organization, and normal mail functionality (send, receive, forward, etc.).