Exporting Data from Evolution

I have been using Evolution for a while and am having difficulty exporting my inbox folder to save on a spare HDD as my IMAP quota is almost full. I can highlight all the emails in the folder but when I click on "save as .mbox" it will only save the first email in the folder not all of them.

Can anybody help me out with this please.

It would have been helpful if there was a local folder option like there is in Thunderbird but I guess you cant have it all !!!