I have also found that you should ask who the customers ISP is. If they are using an ISP, such as AT&T or whoever, and their network is built off this connection - you will have to enter a valid ISP email for this to work.
Example - We recently set up a Savin 8060 and 2824 on demo. At the same location, there was already a Cannon (not sure of the model) there on demo as well. Before I started anything, I asked the customer for a list of any names of users that will be scanning and their e-mail address. I also asked for their ISP and valid ISP e-mail account username. This confused a few people, because their company had their own e-mailed hosted onsite, so they called their head network staffer and had me talk to them. Sure enough, they were using a local cable ISP, but I knew those lines were leased through Comcast. I also knew that Comcast will not allow for our MFP's to send e-mail through their domain into another without a valid Comcast e-mail. When I explained this, the network personal knew exactly what I needed and looked it right up. I then entered their ISP SMTP IP in the for SMTP server name (which was of course different than their Exchange server IP) and their company's ISP e-mail name in the administrators e-mail section (which again was different than their company e-mail) and walla!
When this happens, you of course want to leave off the option to "auto attach sender name".
After I got both of our machines printing and scanning, which took all of 20 minutes, the network person asked how we could make our machine scan to e-mail and the Cannon guys couldn't. I explained, "I asked one more question then they did. That's all." We closed the deal yesterday, I think.
One more quick note... I have found that when customers lease T1 lines or something to that effect, the above rules don't always apply. Most of time, these customers stay within their own domain and don't put a high priority of direct scan to e-mail. These are customers that most likely will utilize scan to folder, ftp or ncp. If they push the issue though, either the above option or Ricoh-ono's might work. I haven't used that method will look into for future reference.