Delivery status notification, invalid header: bad MIME
Solution 1:
There must be an empty line between the message or MIME headers and the message content:
------=_NextPart_b45f3c386576f1955bb5338634f5047f_alt
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: 8bit
Return ID: 1
Return Date: 11/09/2011
Here you'll need an empty line between Content-Transfer-Encoding: 8bit and Return ID: 1.
If Return ID and Return Date should be headers, they are syntactically wrong. Headers must not contain a whitespace in the header name. Valid headers would be Return-Id and Return-Date.