Customer's Exchange server and their Retain server are in different networks and do not point to the same DNS server as required by Exchange's autodiscover service; thus, autodiscover fails to resolve to a user's mailbox when setting up the Exchange module in Retain.
In this case, Retain is connecting to Exchange as a "client" in order to log into the users' mailboxes.
The customer was able to make this work by creating a Conditional Forwarder in the DNS server to which his Retain server was pointing. The Conditional Forwarder record pointed to the Exchange server's DNS server:
This article was originally published in the GWAVA knowledgebase as article ID 2233.