Fix reply-to and missing calendar items after Exchange migration
When you migrate your Exchange organisation, say from 2000/2003 to 2007 using import/export PST (when using a new AD domain, otherwise you can use the move mailbox option), you can run into problems that your end-users can’t reply to old e-mail anymore and/or they see missing entries in calendar items. When replying to an old mail (form before the migration), you probably get a non-delivery report from the Exchange server. When looking at that report, you should look at the X500 address. This X500 address will still point at the old Exchange organization and it still active in the local Outlook client cache.
To resolve this situation, on every user object add the old legacyExchangeDN attribute in your new AD domain as extra X500 address. The Outlook client cache entries will now point to something valid, as all old X500 addresses are now aliases on the new mailboxes.
Mind that the proxyAddresses attribute in AD is multi-valued and you use a qualified tool such as UMRA to bulk-process this data.