I am not aware of any documents describing how to migrate. However, if the customer is taking advantage of features such as content filtering, Message Filter is probably not the right choice, they would want the gateway. If they are strictly doing anti-spam and/or AV, Message Filter would work.
The scope of work depends on a few things:
- What platform are they on?
- What MTA are they using? (SMS for SMTP comes with an MTA or a particular MTA integration? ... I think it might be the same as SBMF)
- What does their bmiconfig.xml look like?
The minimal change would be to convert the configuration to a Message Filter configuration (if they are using an SBMF supported MTA on an SBMF supported platform only using only AS and/or AV).