Normally the usage of Symbian 9 units with OneBridge 5.5 is not problem, since the Hotfix release 5.5. 2007 726
But Nokia has made some changes in their operating system for Symbian 9 units, and therefore you can see behavior of the mail client that due to different versions of Symbian 9 OS will make it more difficulty with the browsing of the inbox/mailbox of OneBridge client.
Our best advise is the keep the old firmware inside you Symbian 9 device untill this bug is fixed by Nokia.