What is a Delta Migration?
A Delta Migration allows the resubmission of a mailbox in order to perform a differential migration pass. MigrationWiz will automatically find data that hasn't yet been migrated, and migrate only that. In other words, a Delta Pass will find and migrate residual data. In most cases, a Delta Pass will complete much faster than the initial pass.
A Delta Migration allows for the performance of an initial migration and then follows up with a background pass. For example, suppose that there is a lot of data to migrate, limited bandwidth, and the need to switch users to a new email system as quickly as possible. Perform a first pass to migrate only the last 180 days of data, and switch users to the new system. Later, follow up with another pass to migrate the rest of the data in the background.
A Delta Migration also allows for a pre-stage of the data. For example, suppose that an email service contract (or software usage agreement) ends in three weeks and it is necessary to move to a new email system. Start migrating the bulk of the data one week in advance. Closer to the deadline, most of the data has been migrated, and a final Delta Pass should complete much faster. In other words, risk has been minimized.
It is possible for the Source email system to receive new email while the migration is in progress. It is also possible for the Source email system to continue receiving new email long after switching MX records (due to DNS replication delays). A Delta Migration is a great way to migrate residuals.