I'm working on this project and the customer wants it to integrate with a groupware package including bulk email, calendars, and task lists. He has heard about Horde and Horde GroupWare and is interested in using that.
Do you have any developer related experience with Horde? If so, then what were your findings? Did you find it to be stable? What about the framework/API? Was it easy to integrate with?
I realise this is a late answer, so your question may've been otherwise answered by now, but:
An application I've been involved with used Horde quite extensively, specifically the webmail aspects. Framework/API/integration wise, it's quite a headache - there's a lot of code in Horde, which makes it unmanageable rather quickly if you do actually have to go in and touch the code frequently.
If what you want is less tweaking, more using, then you've got a powerful tool at your disposal. The company I work for use it extensively for their purposes and are quite happy.
I'd guess that's more comparable to what you want, so I'd say go ahead.