yes, build 303 does work on wm6.1. i tested both on hermes(19949) and polaris(19214). i think it's an issue not of wm6.1 build but of your mmslite setting. there are several setting guides on the internet with different connections, different proxy addresses and different user agents.
this is my setting.
on the connection tab, use softbank (flat rate connection), not wap. i tried wap connection, but there seems to be a file/message size limit per message with this connection.
mms server address: http://mms/
mms gateway proxy: sbmmsproxy.softbank.ne.jp
you must set user agent. in my case, i used V705SH.
user agent: (just copy and paste the two lines below)
i have been using it for a couple of days already, but have not seen any issues yet.