[E2010] [EWS] [Windows] Uploading existing mail (with CreateItem) ends marked as draft

I try to migrate existing mails from a custom mail-system to Exchange. I use CreateItem in order to upload the mails in Exchange folders (for ex. in Inbox). The problem is that the created mails are all marked as draft and when I open one in Outlook Client, I have the option to 'Send' it (plain wrong for an Inbox mail already received from a different person).

How should I make the CreateItem request in order to not have the mail marked as draft? I have already tried various fields (IsDraft, ReceivedBy, extended MAPI properties ...) but no success.

<CreateItem MessageDisposition="SaveOnly" xmlns="http://schemas.microsoft.com/exchange/services/2006/messages"><SavedItemFolderId><t:FolderId ChangeKey="..." Id="..."/></SavedItemFolderId><Items><t:Message><t:MimeContent CharacterSet="UTF-8">BASE64 ENCODED MESSAGE</t:MimeContent><t:Subject>THE SUBJECT LINE</t:Subject><t:IsRead>true</t:IsRead></t:Message></Items></CreateItem>

Thank you in advance for any usefull advice!