Here are the details about the Google Apps for ISP's through Ikano. If you have 5,000 or more email accounts, this makes sense. If you have more than 250,000 email accounts, you can go direct to Google. If you contact Ikano please tell them you saw it on RAD-INFO.net (or contact our office at 813-963-5884 and we can help you with this).
Outsourcing and Leverage
The ROI: you no longer have the power consumption of the mail server and the anti-spam gear; double that because you don't have to cool it either). No gear to maintain. No tech to massage the email server or the anti-spam boxes. No more cost in anti-spam filtering. All that goes away when you outsource it. You time and money and a tech back to work on other projects.
While I am a referral agent for IKANO's program, you should outsource your email regardless to someone: Intermedia.Net, Everyone.net, or GroupSpark. Hosted Microsoft Exchange is a pain to run and most folks want support for our mobile gear (smartphone and laptop), which is nothing more than a pain and expense for you.
Leverage is a smart business move. It means that you leverage the skills, staff, and services of a strategic partner. I know there will be separation anxiety, but you'll get over it... eventually.
One last point:
"Regulations and guidelines like the Federal Rules of Civil Procedure put pressure on IT organizations to ensure that email is properly retained and can be reliably located and preserved in the event of legal discovery. Coupled with the growing importance of email as a store of intellectual property, email archiving has become both legally necessary and critical to the operation of your business." [Ikano] Google does email archiving? Do you? Make sure that your partner does.