I am re-posting this item as I receive a high number of requests for it on the old version of the blog but, obviously, that post no longer exists.
Quite a while ago, I was experiencing an error when trying to use OMA. The error returned by IE was "A system error has occurred" but not due to the usual cause in KB817379
The event log showed the following:
Event ID: 1503
An unknown error occurred while processing the current request:
Message: Index was out of range. Must be non-negative and less than the size of the collection.
The issue only occurred with one mailbox (mine) and all others were OK.
Searching for event 1503 with MSExchangeOMA as the source revealed nothing relevant, but the message "Index was out of range..." lead me (in a roundabout way) to KB319886
"E-mail message is not delivered from Exchange 2000 Server to Exchange Server 5.5 recipients". On the face of it the article appears completely unrelated but it states that individual mailboxes can experience problems if the homeMTA attribute for the user in AD is incorrect.
Following the steps in the article (using ADSIEDIT from the Windows Support Tools) I found that the homeMTA attribute for my user account was indeed incorrect - presumably as a result of the problems I had experienced with having to rebuild my domain after a hardware failure.
Changing this to the correct value resolved the issue and, hey presto, OMA functioned for my account again.