I prefer an off site email service because it really cuts down on the traffic to your site. We have the unfortunate luck of having a DOMAIN.com that is similar to DOMAIN.net.au. It seems like everyone from that domain tells everyone their address is DOMAIN and think people are going to assume the .net.au, but of course they think it is .com
Our SPAM and other junk mail is 99% of the email that would come to our server daily. Our double T-1 line of 3Mbps can't keep up with that much mail and provide Internet acces so we use a company called Appriver.com
The way their service works is you setup your MX records as one of their servers, they get the mail, clean it up, and only pass you the good stuff. You then configure your Exhange or other email server to only accept inbound connections from their IPs and that secures you nicely and frees up all that processing time and storage on your email server.
We have had it for about 5 years now and it works great. They even send each user a nice SPAM report once a day so they can see if someone got caught in the filter by mistake. You can release the message right from the link in the email from them and even put in exceptions for those companies you do business with, but share the same server as some deadbeat company.
It is about a buck and email address.
