The initial question was why MMS worked and now it does not, and they don't even get notices any more. The answer, almost always, is that the provider updated their database and disabled MMS on all N900. So yes, it is in fact as simple as that.

That's quite possible, but again, this is not about some getting through, etc. This is about lack of getting MMS where you once did before.

On a side note: Personally, I wouldn't stay with an operator that's allowing advertisers raw access to their internal network. That's just asking for trouble.

Yes, but the IMEI is broken up by model and vendor as well. Most larger companies (like Nokia) order a specific range and keep a database that they offer to operators that indicates what the model and features each device supports, and what their IMEI TAC or SN range is.

You misunderstand.

When Nokia sends someone a database of which IMEI ranges are for what models, that same database also usually contains data about what each model has for a feature set. Can it do IMS? Does it have Wifi? What frequencies can it use? What compression protocols does it know? It also commonly lists if the model supports basic features, like e-mail, instant messaging, voice call notification messages, and MMS.

Before your provider was saying you had an "unknown device", and probably allows MMS on unknown devices, assuming it supports it. Now they updated their database, and it's saying you have an N900, which doesn't support MMS (out of the box), so they're not sending them along any more.

And yes, other networks may do things differently, so if you roam to another network, they may in fact change if MMS can get to you, based on if they have the N900 listed in their database or not.

The best thing to do is ask the carrier to change the setting for your number. Some carriers can toggle that per user. Smart providers have the system enable it automatically if you send a MMS from the device.