Mail Archiver can archive from AppleMail, Outlook, Powermail, Thunderbird, Postbox, IMAP. It can archive to the Internal Database, Evernote, Filemaker and PDF.
These possibilities are kind of obvious. Sometimes it's overlooked that Mail Archiver also can act as mbox viewer. If you have lots of old mbox, eml/emlx files lying around you can use Mail Archiver to make a consolidated archive out of your messy existing files.
The first step is to go to Setup -> Data Format.
There you select the format of your archive. You can use all formats (Internal Database, Filemaker, Evernote and PDF) for your mbox viewer. Then you select the location.
The other parts of the Setup aren't necessary when you want to archive mbox files. Close the Setup.
Now select "Archive Mbox" from the main toolbar:
You will get a file/folder selection dialog:
You can select just the folder. You can also select individual files of the following types: text, mbox, eml and emlx.
The screenshots shows my folder of test emails. Most of the testing for problems with emails is done with "Archive Mbox" because I can archive individual emails again and again.