Can anyone recommend a perimeter SPAM filtering service? [closed]

I am interested in what folks are using for perimeter SPAM filtering services.

The service must:

• plug in via MX records in DNS (e.g. MX -> perimeter filter -> my mail server)
• can be secured such that only the perimeter service can connect to my mail server
• can be free or paid
• provide a control panel where I can manage my users, alias entries, etc.
• must include filter tweaking (e.g. it must not be up to me to tweak filters)
• user-accessible quarantine

Further Clarification:

• I am not interested in any appliances or firewalls where I have to maintain hardware/software on-site.

PS: Thanks to all who are contributing, I am upvoting answers that provide:

• Clear description of the service
• Tell about their personal experience or evaluation of the service

Postini (aka Google) - pretty inexpensive, not that intuitive to administer but its not too bad.

Microsoft Exchange Hosted Services (aka Frontbridge)

Microsoft Exchange Hosted Services (EHS) offers online tools to help your organization protect itself from spam and malware, satisfy retention requirements for e-discovery and compliance, encrypt data to preserve confidentiality, and maintain access to e-mail during and after emergency situations.

We use MessageLabs and find them to be very good.

I believe they meet all the criteria listed above too.

If you're interested in an appliance, I would highly recommend Barracuda Networks; their appliances do everything you ask and personally it's the best spam filtering I've ever used.