switching mail servers for existing domains

I currently have 2 VPS servers for my business. They serve as mail server and storage server (owncloud including calendar) mainly. I also use them for software development.

I have several questions :

- i want to keep my email accounts for two domains. I can administer my DNS settings so I presume it is possible to switch the email accounts over to mailbox.org. What happens then with the old emails ? Can they be transferred as well ?

- calendar is now based on owncloud. Can this be transferred ? I also sync with my mobile using DavX. This is possible in the future too ?

- storage : i built up quite a big amount of data. What is the size of storage one can get or is it possible to extend the storage capacity ?