Set up a Mailserver

Enable Mailserver

On the Server.app, go tot the tab Mail
Set on.
Edit provide Mail for: and add all domains you need

Configure DNS

On the Server.app, go tot the tab DNS
Select the domain, click on the bottom Create an MX record for this host name

Create User

On the Server.app, go tot the tab Users
Click on + to add a new user.
These are Mac users. Set Email adress (this is for SMTP).
Save
Right click on the user and select Advanced Options...
Add the Email adress as alias (this is for Postfix).

Configure entry on LINK at switchplus

Use Chrome with switchplus.ch (Safari does not work).
Go to Administer Domains. Allocate the domain you want to host with LINK.
Got to Basic Configuration.
Click on the icon on the right of the domain "Edit name server".
Change all mail related CNAME and MX entries to mail.example.com