I added the DNS records that virtualmin proposed to me. Amongst others, there was this DNS record:
MX @ mail.MYDOMAIN.ch priority 5
mail. as a virtual subserver of MYDOMAIN.ch is not existing. After I set this, I could not receive emails anymore. I noticed this now, 20 days later. In every useraccount of that virtual server (Virtualmin -> edit user -> USER -> email settings), there were new emails correctly displayed: (New mail received 10/02/2019 7:42 PM) But the real folder was not containing any new emails.
Now I've removed the DNS record and I can receive emails normally again. But is there a solution in getting these Emails that were sent to the wrong subdomain? Apart of that, I don't really get it what the point of that subdomain MX DNS record would be. could someone elaborate?