Which products can be used as managed email list with external entities?

Currently we are using Lyris ListManager Server 11 as our email list manager to communicate and share emails. We are not using ListManager as email marketing tool, but rather as communication tool with external partners / entities.

Is there any product / addon for MS Exchagne Server which would allows us to host email lists internally and still let the external entities / partners to subscribe / unsubscribe to lists?

Thanks