sorry i do not have n900
after asking the same as your question in the community i have had no answer so far. i know on tmobile usa you can actually put the number like say 5555555555@tmomail.net and send as an email and on the regular phone that will come up as a mms i have asked that too but have not had any answer on that either.
i do remember that your question of what happens when some one else sends you a mms? had been asked before but there was no concrete answer on that either.
hope fully one of those 300 who have a loaner can help us.
or when i receive mine i will PM you can we can check