Oh, I thought 1GB where minimum but I see now that 2GB is required for production use.
I guess I will have to find another email solution.

Thank you for your support.

1.5GB.

Also I forgot to say, /proc is mounted.

proc on /proc type proc (rw,noexec,nosuid,nodev)

When I start zimbra starts flooding whit

Error: /proc must be mounted
To mount /proc at boot you need an /etc/fstab line like:
/proc /proc proc defaults
In the meantime, run...