MAPI Programming, ConfigureMsgService causes hang

This question concerns the use of MAPI programming to connect to a Microsoft
Exchange 2007 server.

Can anybody think of a reason that ConfigureMsgService would cause a hang?  We have
an application that creates multiple processes in the context of a service account
that has all permissions to the server.  The process goes through creating a temporary
profile and configuring it for a given server name and user id.  This code had
been working fine to my knowledge.

Now something has happened, that is causing *most* of these processes to fail, at
the point of ConfigureMsgService.  It just stops, no error code.  I believe Exchange may have
popped up a dialog box asking for authentication (I'm not sure at the moment, waiting
to get confirmation).  The strange thing is, some accounts will work, and some don't.
If we retry again with an account that works, it might fail, and vice versa.  It appears to be
random.  If we are getting an authentication dialog, why is it not consistent?  Is
there any way this behavior can be turned off?

Any ideas?  Suggestions very much appreciated.