HI,
details
server debian |open fire 3.5.1
clients mostly pandion, but some PSI, spark too
We frequently get reports that message are being sent but they never arrive. we use iball and can see in the logs that the message has been sent, the serverlogs it. I dont know how to recreate it, it just does it, I have had first hand experience. The client that cannot get the messages can almost always send mesages ok, and they arrive. If i get the client to log out of the IM close it then open/login again it usually fixes it.
However users here are loosing confidence in the
server, they cant be sure the message is getting to the user, so
they are all emailing or foning them kinda defies the point a lot! 
we use pandion here mainly however it still happens regardless of
jabber client. the mac boys use psi and there are still a few spark users
out there .all report the problem.
The problem has been reported since we first installed it when it was still wildfire. Since then we have tried a fresh install of the server, purging all files associated with of/wf adding more ram to the server, new database, moving the database onto the openfireserver and removing all plugins too.
My config is attached and was generated using the openfire setup, no changes have been done manually, any help appreciated.