Some time ago that already we speak that the Nokia N900 had not support for MMS, but now, thanks to the application based on Python called fMMS, already is possible to send MMS from our N900.
fMMS is not yet available in the repository Extras-Devel, but if you can install as .deb. Personally i don’t like install .deb files, but if you are very interested in prove fMMS is a risk that you can run or wait it in Extra-Devel .
After you download and install fMMS you will have to configure it, for this creates an access point of your operator to include the proxy of the operator and connect manually as MMS APN.
Some FAQS of the author:
Solution: Make sure you type in the name EXACTLY like it says in “Internet Connections”. This means it ONLY TAKES EXISTING APNs – it does not create a new one.
Solution: Make sure you are connected to the MMS APN (you have to connect manually) – if you still get the error, try pinging the mmsc host from the terminal. If it still fails, double check your connection settings. If your provider list a “MMS Proxy” or “MMS Gateway”, input this in the HTTP Proxy settings for the connection
DON’T FORGET TO SET THE PROXY IN INTERNET CONNECTIONS