Downloading incoming and outgoing email to another server

Hi,

I have VPS running with CWP. There are abount 50 email user and quite a lot of emails with attachments are being sent/recieved. This is causing the VPS getting filled up.

keeping the email and attachment is important. What can I do other than buying more VPS space?
How can I move the emails somewhere else , maybe locally hosted mail server ?
We have a Synology NAS in the local network. Is there anyway we can backup emails into it ? still should be searchable.

Thanks for your advice. Have a great day ahead !