_MailItem.Save method

Saves the Outlook item to the current folder or, if this is a new item, to the Outlook default folder for the item type.

Namespace:  Microsoft.Office.Interop.Outlook
Assembly:  Microsoft.Office.Interop.Outlook (in Microsoft.Office.Interop.Outlook.dll)

void Save()

If a mail item is an inline reply, calling Save on that mail item may fail and result in unexpected behavior.