My VSTO plugin intercepts attachments, however there’s a case that when I attempt to intercept attachments (on-send) from the explorer I receive the error at the very end. The logic is entirely the same for Explorer and Inspector attachment handling, it looks as:

currentMail.Attachments[i].SaveAsFile(tempfile);

The error is:

Cannot save the attachment. Cannot add the attachment; no data source was provided.

Now I suspect this is related to the email being in use (by Outlook), as I am able to trigger the same code if I modify it as a draft first, save it and then send the email.

What does error code -2147467259 represent though?

More of the error:

Data    {System.Collections.ListDictionaryInternal} System.Collections.IDictionary {System.Collections.ListDictionaryInternal}
ErrorCode   -2147467259 int
HResult -2147467259 int
HelpLink    null    string
InnerException  null    System.Exception
Message "Cannot save the attachment. Cannot add the attachment; no data source was provided."   string
Source  "Microsoft Outlook" string
StackTrace  "   at Microsoft.Office.Interop.Outlook.Attachment.SaveAsFile(String Path)rn   
TargetSite  {Void SaveAsFile(System.String)}    System.Reflection.MethodBase {System.Reflection.RuntimeMethodInfo}