DEVONthink is the repository for emails you want to save.

Its hierarchical groups structure, automatic classification function, and powerful search engine are perfectly suited to adding, sorting, and finding emails in your databases. DEVONthink comes with advanced email archiving functions that transfer selected messages or mailboxes from Apple Mail, Microsoft Outlook, and any email applications using standard Unix mailboxes, e.g., Thunderbird, to a DEVONthink database. The original structure of the message, as well as all attachments, are preserved when possible. You even can reply to messages directly from DEVONthink's contextual menu commands!