Discrepancy between Total items and migrated items during the migration of a mailbox
At the completion of a mailbox migration, you might notice a difference between the number of migrated mails and the total number of items in the source mailbox.
What is the reason?
There are several reasons for this and this doesn’t mean that there are missing mails.
1. Some items are ignored.
The platform migrates mail. Items stored in a mail folder and that are not classed as mail will not be migrated (for example, a calendar invitation object that is in the inbox or the trash). This explains why at the end of a migration, the total number of items migrated is not the total number of items in the source mailbox. (In this scenario, however, the calendar appointment will be migrated correctly and will appear in the calendar view).
2. Some mails are duplicated.
The platform supports incremental passes and delta passes.
When it detects a mail that has already been migrated, it will not push it a second time.
The mechanism is based on the messageIDs of the mails.
Sometimes, you receive mails twice — the first time because it is addressed to you and the second time because it’s also sent to a group you are member of.
This is particularily the case of you send a message to a group you are member of: The mails will be stored in your SentItems folder and will also appear in your inbox.
During the migration, the platforms migrates the first mail and for the second mail, then it detects that the messageID has already been migrated and does not migrate it a second time.