Certainly most hosting providers have their own hosting solution. Nearly every data center provider that I've met with has a host solution based on one of the in-house solutions you might buy. For example, if you're really keen on buying Symantec Corp. or Mimosa Systems Inc., it's pretty likely that you'll find a service provider that is offering that solution as a managed service. The first thing to do would be to look at your current business partners for hosting services and you'll probably be pleasantly surprised that they have an email archiving solution that they can roll out pretty quickly.
The next step would be to look at more of the dedicated service providers. Of them, Zantaz (now owned by Autonomy Corp.) is the one that comes to mind immediately because they have built an entire hardware/software solution around hosted/managed service email archiving. They of course also offer products in-house, but they come immediately to mind when you mention email archiving.
There are a lot of other options as well. There are a lot of smaller companies that have their own solution, which is not someone else's solution that they just rolled out in their data center. And there a lot of niche products that focus, for example, on the financial services industry.
Then there are the big guys, Google, Yahoo and Microsoft. All of them offer hosted email archiving as a managed service and all of them have pretty compelling products and prices. As they do for so many other internet outsourced applications. Those are really the ones that I would start with. Think about if there is a potential with your current hosted provider. Then think about whether you need a specialist or if you need more a general application like Zantaz. Then think about the big guys in the industry and whether they can do what you need.
Check out the entire Email Archiving FAQ.