I have used Messaging Architect's M+Archive for Exchange & GroupWise (no lotus notes support). It is free for companies 200 or less, and I think they *might* be expanding that to 500. Another one a coworker has used a lot is Gwava's Retain for GroupWise, works great. Gwava has also recently brought out their exchange version of the product as well. Both have great policy/job options, eDiscovery, integration into the "fat client" & into the web client as well. One thing you might also want to look into is Gwava's Retain for Blackberry enterprise server, it allows you to archive everything going on with blackberries (and now that RIM has announced upcoming iOS/Android support, who knows, you might be able to archive them in the future with retain for BES as well?!).
I would like to know what experience people have had with Sonian cloud...