We are struggling with Exchange API's to find the right way to import msg and eml files into a Group mailbox. The scenario is that in the past emails was archived in SharePoint libraries and in the event of moving it all to O365 Groups we really would like to get the mails back into the Group mailbox.
Anyone has an idea to which API to use - this cannot be a manual process as we have 30000+ Groups to manage and hence automation must be applied. I can see tools being able to do this - none of them have any scripting capabilities as far as I can see.
EWS should work fine for this scenario, and so should the Graph API. I'm not aware of any tool that specifically handles importing MSG/EML files to Groups though, but there are multiple tools that can "migrate" from a regular mailbox to Group, so perhaps you can use this as a workaround.