Exchange Server 2007: Troubleshooting Move-Mailbox operations that fail due to "Being Too Long" attribute

You may experience problems performing mailbox move operations if the number of characters on a given Exchange attribute is greater than the default values defined by /SchemaPrep. Most commonly this issue occurs because of an administrative and unsupported edit of the Schema to allow for more characters to be present on a given user attribute. […]

You may experience problems performing mailbox move operations if the number of characters on a given Exchange attribute is greater than the default values defined by /SchemaPrep. Most commonly this issue occurs because of an administrative and unsupported edit of the Schema to allow for more characters to be present on a given user attribute. Afterwards when the Mailbox-Move is attempted new Move-Mailbox prevalidation logic runs and ultimately fails the move if the user has a larger number of characters present on the object than that which was defined by /SchemaPrep for the attribute. This error will only manifest itself when the source or target of a mailbox move is an Exchange 2007 mail store.

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