When synchronizing source and target mailboxes and getting ready to switch there is a discrepancy in the amount of items and size of mailboxes between the Source and Target when looking through the Exchange System Manager.
This applies to migrations using the MAGE agent also.
The Exchange System Manager reports the total amount of items in the mailbox including all messages and calendar items. Ensure that the mailbox is included in both mail and calendar collections. If it is, check mail and calendar logs for errors related to the particular mailbox.
Small discrepancy is possible due to some corrupted messages and attachments being skipped during mail and calendar item migration.
Here are some more helpful tips:
1. In some target environments policy may be configured to clean Deleted Items mailbox folder and delete messages after certain age. This may cause discrepancy and appear as missing mail when comparing source and target mailboxes.
2. Deleted Items column can also be added when checking and comparing total mailbox items in ESM (Exchange System Manager), View - Add/Remove columns .... , Deleted Items (KB). This column is not the same as "Deleted Items" folder in mailbox (from #1) but reflects amount of soft-deleted items that are kept in Exchange Information Store according to Retention policy. The Deleted item retention time [days] field controls how long items stay in the dumpster before Exchange deletes them finally and irrevocably. Amount of messages displayed under Deleted Items (KB) column is not counted towards Total Items column and there is normally no need to compare them, just looking into Total Items would give a good idea of source and target mailbox sizes.
3. The MAGE does NOT Synchronize any folders that are level with the "Top of Information Store" Ie. Blackberry, Cisco, Dumpster Data/Recoverable items, and other hidden folders. These are enumerated into the product statistics for source item count, but are NOT synchronized to target. Currently there is NO way to skip the dumpster check.
4. In some situations a target may have logged onto their target mailbox incorrectly and deleted items or folders, which affects the migration of messages. Logging onto the target mailbox by a user before switching is not recommended, and can cause multiple problems during mail sync.