Exchange 2007 corrupt edb

We are backing up mailboxes on an Exchange 2007 server and have recently started seeing a backup error on one mailbox out of about 25.

T-597: PtSend: cmd MbeF/msg RdBg: byte count 1041 exceeds maximum of 1024
Trouble reading files, error -523 ( service transaction error)

Anyone know what this means or what we can do to resolve it?

Exchange is on a different server than Retrospect. We are running Exchange 2007 with the latest service packs and updates. Retrospect is version 7.7.620 and the server with Exchange has client version 7.7.114. Let me know if you need any other information. Thanks.