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In outlook 2010, a scheduling user is creating an appointment and sending via exchange 2010 to a users yahoo.com email account. <br />
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I Ran the Microsoft fix it from <a href="http://support.microsoft.com/kb/958012" rel="ugc">http://support.microsoft.com/kb/958012</a>&nbsp;on the computer with outlook, and manually changed the outlook mail setting to plain text. neither resolved the problem. <br />
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Is the issue in Exchange server?
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In outlook 2010, a scheduling user is creating an appointment and sending via exchange 2010 to a users yahoo.com email account. 

I Ran the Microsoft fix it from http://support.microsoft.com/kb/958012 on the computer with outlook, and manually changed the outlook mail setting to plain text. neither resolved the problem. 

Is the issue in Exchange server?

winmail.dat appears to a user's yahoo account when an appointment is sent to them.

An email client, email reader or more formally mail user agent (MUA) is a computer program used to access and manage a user's email. A web application that provides message management, composition, and reception functions is sometimes also considered an email client, but more commonly referred to as webmail.

Internet / Email Software

Within Internet message handling services (MHS), a message transfer agent or mail transfer agent (MTA) or mail relay is software that transfers electronic mail messages from one computer to another using a client–server application architecture. A MTA implements both the client (sending) and server (receiving) portions of the Simple Mail Transfer Protocol (SMTP). The terms mail server, mail exchanger, and MX host may also refer to a computer performing the MTA function. The Domain Name System (DNS) associates a mail server to a domain with mail exchanger (MX) resource records containing the domain name of a host providing MTA services.

Email Servers

A server is a computer program or a machine that waits for requests from other machines or software (clients) and responds to them. This architecture is called the client–server model. The clients may run on the same computer or may connect to the server over a network. The purpose of a server is to share data or hardware and software resources among clients. Typical computing servers are database servers, file servers, mail servers, print servers, web servers, game servers, and application servers.