I'm playing Monday morning quarterback so take this with a grain of salt and know that I am not perfect and have made many mistakes like this in the past and still continue todo so from time to time.
I know the customer is always right, but sometimes you have to take back the posture. In hind sight, maybe you should have said we will come onsite once we have the mail server settings. Remember, even in sales sometimes you have to say No!
It seems the issue you are dealing with is because the IT vendor/support was not involved early enough in the installation. They should have easily been able to provide the necessary information for scan to email in a Hosted Exchange environment.
Not building a good relationship with the IT vendor early is a recipe for frustration and customer dissatisfaction. I have learned this through similiar situations but still need it reinforced from time to time as we get busy and think everything will be okay.
The IT "partners" I deal with would prefer a call a few days prior to install to let them know what the plans are and become allies, worker bees and lead sources when treated properly. When not involved early, they are venomous, frustrating, arrogant and controlling in the end ruining my day, the install and the relationship with the customer.
If/when we do not contact them like we should or if they do not want to play well we have 2 mail servers that we use on behalf of the customer so that we can deliver what we promise.
Iris, correct me if I'm wrong, but even if you use a solution such as Udox, you would need to have the proper information for the Hosted Exchange server. There is no substitute for preparation.