After a mail migration to a new domain/Exchange organization using Exmerge, you will lose the ability to reply to old email and modify old calendar items. The reason this occurs is because the message is stored with x500 address from the old domain. In order to correct this you need to add this old X500 address as proxy address for the user account in Active Directory in the new domain. You will need to build the user's new X500 proxyAddress from the the user's LegacyDN attribute from the old domain. To get this attribute, launch ADSIEDIT.msc on the old domain controller and open the properties of the user account. Locate the legacyExchangeDN value and copy it to notepad.
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