When sending an iTip reply as a delegatee of the event invitee I got in a reply from the server:
The message WAS NOT relayed to: <test2.test@example.org>: 554 5.6.0 bounce, id=26465-07 - bad header This nondelivery report was generated by the program amavisd-new at host kolab.example.org. Our internal reference code for your message is 26465-07/3uZqARq-cfif INVALID HEADER Duplicate header field: "Sender"
I confirmed with SMTP debug that the send mail does not contain the duplicate. It contained only one Sender:
Sender: "Manager, Jane" <jane.manager@example.org>
However, the error report says the message headers was:
... Sender: jane.manager@example.org ... Sender: "Manager, Jane" <jane.manager@example.org> ...
I guess the duplicate was added by smtp access policy module. Maybe it just does not handle Sender address with name correctly?