As per Microsoft: "This Warning event is logged when Microsoft Exchange Server cannot download one or more entries of an offline address list". See MSEX2K3DB
for additional information about this event.
From a newsgroup post: "To resolve this issue, you must increase the diagnostic logging on the OAL generator. See ME311742
for troubleshooting the Exchange 2000 Offline Address List".
I solved the problem by reducing the length of the E-mail adress in the distribution list.
Some of the addresses did not get generated in the offline address book. To see which ones, go to the properties of your Exchange server that has the OAB, to the Diagnostic Logging tab, and under MSExchangeSA, set the logging level for OAL Generator to Medium. Then regenerate the address book by going to Recipients -> Offline Address Lists, right-click on the address list affected and select Regenerate. Now, view the application event log. In my case, the error was caused by a mailbox on an Exchange 5.5 server that only had an x400 address and no SMTP address.