Is there any chance of implementing a MAXPERUID variable
of some sort, to limit the total number of running "imapd"
processes under a particular uid?
This is a system function. On Linux, this can be set with 'ulimit -u'. Note
that this affects all non-root processes, so it is going to apply to all
non-root processes. ulimit -u needs to be set in the script that runs
Courier-IMAP's startup script.
Note that a separate process, couriertls, exists in encrypted connections,
so a ulimit of 10 processes may actually permit a maximum of 5 encrypted
connections at the same time.