A full text index must be created for every mobile enabled mailbox in order to receive correct results on IMAP queries.
The full text index should be updated every time a new messages is delivered to the mailbox. For this reason, "index update frequency" should be set to "immediate".
Pleas note that not all Domino versions - especially older versions - work as expected, even though all settings are configured correct. As a matter of this, not all actually present messages are sent by ixi-UMS Business. In this case, the full text index must be updated like shown below.
More information about "updall" and its parameters can be found in the IBM Lotus Domino Administrator help.