Archive from any IMAP account.
Mail Archiver X can archive mbox files without any email client.
The built-in browser has the familar 3-pane interface of every email client.
You can archive directly to the Internal Database, Filemaker - if you have a Filemaker license - and PDF. If you archive to the internal database you can export the complete or partial data to Filemaker, PDF or MBOX.
Each format has options to tailor the export to your needs. The screenshot shows the options for PDF.
If you have a Filemaker license then you can use Filemaker for your archive. Filemaker versions 16-18 are supported.
It's just a bit annoying to remember to do something. Archive daily, weekly, monthly or once a quarter.
Mail Archiver is available in: