We recently merged with another company and have a trust on our domains. They can both send e-mails and calendar appointments to each other with no problem. Recently USERA in DOMAIN1 sent a calendar appointment to USERB and USERC in DOMAIN2. USERC did a "Reply to All" when he received the meeting request. The message was delivered as a new calendar appointment from USERA to both USERA and USERB. This lead USERB to believe this was a message from USERA and all kinds of interoffice shenanigans ensued. The first dead give away was that the e-mail header showed his message came from DOMAIN2.Header Excerpt 1--------------------------------Received: from exchange.DOMAIN2.corp ([1.2.3.4]) by exchange.DOMAIN1.corp with Microsoft SMTPSVC(6.0.3790.1830);--------------------------------Although the message truely believed it was sent from USERA, the second give away was that USERC was not listed as a recepient since he executed a "Reply All".Header Excerpt 2-------------------------------From: "USERA" <USERA@company.com>To: "USERA" <USERA@company.com>, "USERB" <USERB@company.com>Return-Path: USERA@company.com-------------------------------Challenge Question #1: What would cause this?Challenge Question #2: What, if anything, can be done to correct this from happening in the future?