Best way of merging mbox files

[Marc Roos]

When concatenating mbox files like described here 
https://xaizek.github.io/2013-03-30/merge-mbox-mailboxes/. You will end 
up with an 'unsorted' mbox file. Is this going to be a problem 
esspecially when they are large >2GB's and new emails will be written to 
it?
The email client nicely sorts the message from folder A "foldera 5 last" 
as last, but of course the mbox is not like this. 

Is there a better solution for merging files?

Having:

A folder A with messages:
From	Subject	Received	Size	
test	foldera 1	16:18	665 B	
test	foldera 2	16:18	665 B	
test	foldera 3	16:18	665 B	
test	foldera 4	16:18	665 B	
test	foldera 5 last	16:29	670 B	

A folder B with messages:
From	Subject	Received	Size	
test	folderb 1	16:23	665 B	
test	folderb 2	16:24	665 B	
test	folderb 3	16:24	665 B	
test	folderb 4	16:24	665 B	
test	folderb 5	16:24	665 B	

[@ mail] cat .foldera .folderb > .folderc

Getting a folder C with messages:
From	Subject	Received	Size	
test	foldera 1	16:18	665 B	
test	foldera 2	16:18	665 B	
test	foldera 3	16:18	665 B	
test	foldera 4	16:18	665 B	
Mail System Internal Data	DON'T DELETE THIS MESSAGE -- FOLDER 
INTERNAL DATA	16:19	454 B	
test	folderb 1	16:23	665 B	
test	folderb 2	16:24	665 B	
test	folderb 3	16:24	665 B	
test	folderb 4	16:24	665 B	
test	folderb 5	16:24	665 B	
test	foldera 5 last	16:29	670 B