You may receive the following error when attempting to access OMA after performing a swing migration: "A System error has occurred while processing your request. Please try again. If the problem persists, contact your administrator"

Additionally, event 1503 from source MSExchangeOMA will be logged on the server.

You should notice that OMA works for new users, but not those who have been migrated. This is because of the HomeMTA attribute in Active Directory.

Use ADSIedit to confirm this is the case, compare a new user to an old one - you'll need to update all of the old accounts with the new value.

Be careful when using ADSIedit! This is like regedit for AD, if you mess something up you may have to start all over again!

This is documented in Microsoft KB319886, which talks about Exchange 5.5 and 2000 but is applicable to Exchange 2003 and Small Business Server 2003.

Source: http://forums.techarena.in/small-business-server/570415.htm