At the completion of a mailbox migration, you might notice a difference between the number of migrated emails 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 emails.
Some migrated 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).
Some emails 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 messagesID of the mails.
Sometimes, you receive emails twice — the first time because it is addressed to you and the second time because it’s also sent to a group you are a member of.
This is particularly the case if you send a message to a group you are a member of: The emails will be stored in your SentItems folder and will also appear in your inbox.
During the migration, the platforms migrate 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.