Mail Archiver X is a Mac app from Moth Software Mainz (Beatrix Willius) for collecting and archiving into a database email from popular email apps – Entourage, Outlook, Eudora, Apple Mail, Powermail, Postbox, Thunderbird – and it can also import standard mbox files or collect email directly from Gmail, Office 365 Exchange or iCloud. Archived email can be exported in mbox format or to PDF, Evernote, text, FileMaker or SQL formats. Other features include a built-in email viewer, a scheduler, and a template for using FileMaker instead of the internal database, if you prefer.
Mail Archiver X 5.2.0 is priced at $44.95 for OS X 10.11 and up with a 10-day trial period. (Older versions are available with support for earlier Macs.) Multi-computer and multi-user licenses are more expensive. The latest version includes performance and user interface improvements.