Mail-In-A-Box and OpenVZ normally do not play well together. For some reason, Time4Vps was an exception. I used them for 2 years … granted, I had to make some tweaks to the initial OS as it was an extremely stripped down version of Ubuntu compared to say Digital Oceans install, but it worked.
Then I was forced to move from Time4Vps. I used other OpenVZ providers with horrible results until I discovered that OpenVZ was not suitable. I wish that there was some guide or documentation to that fact as it would have saved me hours of grief!
I guess the point that I am trying to make is that OpenVZ should generally be avoided for Mail-In-A-Box installs.