about add lots additional mailboxes

Hey guys，I have a problem now。My customer has to add many mailboxes to her outlook  for work（just add，not profile logon），now，she can not open these additional mailboxes，even her personal mailbox can not too

This problem happened with some guys，Is this about Mapi sessions？Over the Mapi sessions limit？

Hope someone help me，thanks a lot

PS：The environment is exchange2010+outlook2003