Interesting point that the Nokia phones are not supported since Nokia is listed as one of the company's that originally developed the SyncML protocol specifications.
There is a company called Synthesis that developes SyncML servers and clients and SDK's for many platforms. Their SyncML products, as well as applications based on their SDK's, do not seem to be supported by SmarterMail either. SmarterMail rejects connections from them with the same message that you are seeing on your Nokia phone.
Note that in this instance, my two devices are an iPhone 3G and a Blackberry Curve. I would love to get rid of the Blackberry if the iPhone syncing with Contacts and Tasks were supported by SmarterMail.
Being able to sync calendar, contacts, and tasks with SmarterMail was the single biggest reason for purchasing SmarterMail Enterprise. But since Funambol seems to be the only client supported by SmarterMail for SyncML, and Funambol does not sync Calendar or Task items on the iPhone, it seems to have been a waste.
Eddie