One of the following three status messages will appear in the ODM mail migration events. Only the "Preparing for MailSync Migration" status indicates that mail sync is enabled.
Preparing for FullSync Migration - "FullSync" indicates that this is either the first initial sync of the mailbox, or that the Reset Migration feature was enabled in the mailbox migration options. Either way, this status message indicates that no mail items will be synced (updated, deleted, or modified) in the target mailbox, as "FullSync" assumes the target mailbox is empty. Instead, this "FullSync" will use the "Simple" migration in engine in ODM to perform a full copy the initial data and collect the current "sync state info" to enable the "MailSync" engine for future migration passes.
Here is the formula:
"FullSync" = "Simple mode" + collecting the current sync state info for future mail sync sessions.
Preparing for MailSync Migration - This status message indicates that mail sync is enabled and that a prior FullSync has completed successfully. This migration pass will update items that have changed, and copy new items, in the source mailbox to the target mailbox.
Preparing for Simple Migration - This indicates that mail sync is not enabled for this migration pass. Instead, the old simple copy engine will be used. ODM will disable the mail sync feature if either the Migrate to custom folder or Only migrate mail within specified date range or Migrate all folders except options are selected in the mailbox migration wizard.
Please see this post for additional details on the ODM Mail Sync feature.
Please see this post for additional details on the Reset Migration option.
