A company that I maintain the e-mail server for has been using PeertoPeer.net for off-site anti-spam, virus scanning and anti-phishing protection. Don't be put off by the sleazy marketing approaches on their website. After 3 years of use, I'm happy to report that the only experiences I've had with them have been:
- 100% uptime
- Friendly, prompt, and technically adept tech support, U.S. based, not off-shored
- Effective filtering that has not required any interaction for tuning, false positives, etc.
- Affordable service - $19.95 per domain
- Their filters have an excellent cross section of sample data with which to recognize malicious e-mail, which makes them very effective. I don't remember having any false positives in 3 years (and yes, I check the spam quarantine regularly).
- It reduces your drive space, processor, memory, and bandwidth consumption - bad mail that is filtered never hits your mail host.
- It makes for a leaner security profile - at your firewall, you can lock down the hosts allowed to connect to your mail host to only peertopeer.net servers. This is a huge benefit in terms of protecting your server from the badness of the open internet.
- It works with any MTA server, not just Exchange (a major cost savings right there).
- Only one proven, robust protocol, SMTP, is employed for mail to come into your server. Not POP3, IMAP, or strange proprietary protocols for mail retrieval to your MTA, a function for which they are not designed.
- The cost of $19.95/mo per domain is slightly higher than a subscription to mail host plugin service, such as GFI mailessentials, for a domain. However, this minor premium is quickly nullified the first time someone doesn't spend hours fixing a broken Exchange plugin when it breaks (not to mention the mail downtime). As for the individual spam viewers available for $49.95/mo, I have not found these to be necessary, as after 3 years, there are never any false positives.
- Probably the most important, and unsung feature: They queue your mail when your host goes down. This shields outside servers from any mail server outages due to maintenance, etc. on the customer's mail server. Believe it or not, these days, a deferred /failed message can have a considerable impact on the professional reputation of an organization.
There are lots of ways to deal with spam, virus, and phishing e-mails, but after 3 years of analysis, I'm settled on the best, most cost effective way to deal with it. For my money, it's peertopeer.net