After performing a mail-only mailbox migration, you may notice that some folders are of a different folder type (i.e., mail folder instead of calendar folder) in the Destination, and that some items may be missing. This happens when mail folders contain non-mail subfolders, such as calendar or contact subfolders.
In the example screenshot below, Folder 1 is a mail folder, and Folder 2 (a subfolder of Folder 1) is a calendar folder in the Source environment. If you perform a mail-only mailbox migration, then Folder 1 and its subfolders are migrated. However, Folder 2 is converted to a mail folder in the Destination and calendar items saved within that folder are missing post-migration.
To circumvent this issue, we recommend that you direct users to separate folders of different types and move non-mail folders to the root. If separating folder types is not possible, we recommend that you perform a full migration of all mailbox item types (mail, calendar, contacts, etc.).
This issue can also occur when using the folder mapping support option to map the contents of a Source mailbox into a single folder on the Destination. Read the Can I migrate a Source mailbox into a single subfolder of a Destination mailbox? article for more information about how to circumvent this issue.