Ubuntu 14.04 (released in April 2019) is end of life as of April 2019
So Cannonical doesn’t update your system anymore, this could pose a security risk.
If you want to read more about this:
You cannot upgrade so you don’t have to read complicated procedures:
The only supported way to update an existing Mail-in-a-Box to Ubuntu 18.04 and version 0.40 is to backup + restore to a new, clean Ubuntu 18.04 box
See Mail-in-a-Box version v0.40 and moving to Ubuntu 18.04
Wait for a rainy day and move your stuff to a supported box!