controlling STARTTLS by IP address

[Michael Fox]

On my POP3 server, I need to be able to control the use of STARTTLS by
client IP address.  Specifically:

* Clients on certain internal subnets (e.g., 192.168.1.0/24) must not have
the option to use TLS.  If the client tries to use STARTTLS, the option
should be rejected.  This is to satisfy US FCC rules regarding the use of
encryption over certain radio frequencies.
* All other internal clients (e.g., 192.168.0.0/16, but not 192.168.1.0/24)
should be able to use STARTTLS if they choose to.
* All external clients (0.0.0.0/0) will be required to use TLS.

Is there a way to control which clients are allowed to use STARTTLS
according to the client's IP address?

Thanks,
Michael