I gave up on any of the conventional email setups as they are all far too slow and labour intensive as well as they don't have enough features, and some are just plain UGLY.

I've moved to Google Apps for all my clients, and I use the Google Cloud Servers to route all the emails which has been extremely reliable. Avoiding over half of my clients complaints about the WHB Server issues of late.
As long as the emails continue working they are relatively unaware of a lot of site issues which is a lot of help to me.
Yes, there are limits to the emails. but most commercial clients don't notice this as they all use standard POP3 collection methods, such as Outlook to collect mail anyway. So they don't leave enough on the server for the limits to be any issue.
They all just LOVE the ability to run group domain emails quickly and easily.
Setup is a breeze.
They can run group calendar, docs and email apps using their own domain.... ie. calendar.theirdomain.com and mail.theirdomain.com and docs.theirdomain.com
It also allows them a comfortable portable email solution and I have setup many clients iPhone's to connect while they are OOTO.
The Auto-Responder is fantastic while they are away too.

You just can't beat the best for online email, and knowing you are using Google's Cloud, well you don't get more secure than that do you!

I'm in love with it hahahahaha