I don't have a fix and wish there was an entry here with a fix but I can tell you that this error is not caused because there is no Exchange server around.
We are getting this error on a system that is looking for an OpenLDAP directory. I can do an ldapsearch of the directory on the remote server but Webmail wont show the entries.
ldapsearch -xh my.other.server -b dc=company,dc=com
and this returns (this is just the initial data I have not included the 100s of entries returned):
# base <dc=company,dc=com> with scope subtree
# filter: (objectclass=*)
# requesting: ALL
# External, company.com
description: External contacts/suppliers with requirements for LDAP entries
Which is enough to tell me the ldapsearch can reach the other server where theOpenLDAP is but Webmail isn't displaying the data? Hence my saying that it isn't related to Exchange or not limited to Exchange.
But why it is occuring I don't know.