I thought, let's give WT5 a spin. I signed up for a VPS at Arubacloud and chose a CentOS7 image. SSH-ed into the new server, did a yum update and installed NethServer7.
Then added LDAP account provider and installed WT5. Added DNS for the domain at my registrar.
It took like 10 clicks and a cup of coffee and I had mail running. Still need some 'getting around' with webtop, but at first sight it is looking good and running smoothly.