Sounds to me like the 5.5 servers are not replicating properly.The SRS acts like a pseudo 5.5 Exchange dir.edb that can communicate via the ADC with Active
Directory. If you do an admin dump of the user in 5.5 and an LDP dump of the same user from AD, do all the attributes match? In particular look at homemdb, homemta, and proxy address. To do an admin dump in 5.5, go into 5.5 Admin in raw mode. This can be done by typing admin -r from a command prompt in the directory where the Exchange Administrator program resides.
Also verify that your configuration agreements are set to two-way and not one-way.