Although I haven't used the feature myself Poco does have a multi-user facility.
This works with a single copy of Poco rather than multiple copies. Each user's messages are stored in
their own directory tree. You can configure Poco to start with a specific user, prompt for a user or
setup multiple icons each with their own command line parameter to start with a specific user
Since the messages are stored as plain text you would have to encrypt them to prevent someone else
from viewing them (Poco does support encryption, see the "Privacy" pane of the "Account setup"
window) or use external security to prevent access.
If your operating system allows you to assign permissions to folders then this is the ideal way to
implement this type of security.
If you prefer to have a separate PocoMail install for each user you can also do that - just install
PocoMail into separate directories and run each for each user.