Question : Why any word or excel file attachment converts in to winmail.dat format?

I have setup sendmail on RHEL 5 with MailScanner. All is working fine but from last week i am getting some problem with one user, when he is sending mail with attachment of any excel or word file it convert in to winmail.dat format at receipient end.

Sender is using ms outlook 2007 but previously he has ms outlook 2003

Receiver is using Mail Application based on MAC OS 10.5.

Please suggest me what could be the problem. i have changed nothing at server end.

Answer : Why any word or excel file attachment converts in to winmail.dat format?

Please have a look in this...

To resolve this problem, use the plain text format. To do              this:              

1. On the Tools menu, click Options, and then click Mail Format.  
2. In Compose in this message format, click                        to select Plain Text, and then click OK. 

NOTE: To send to certain recipients that use RTF format and others              recipients that use plain text format, you must set the option for the              recipient in either the Personal Address Book or the recipient's contact              record.

Use the following steps to remove the RTF format from the              recipient attribute in the Personal Address Book:              

1. On the Tools menu, click Address Book. 
2. In Show Names From, click the Personal Address Book. 
3. Select the addressee that you want to set as plain text,                        and then click Properties on the File menu. 
4. In the SMTP-General tab, click to clear the Always send to this recipient in                        Microsoft Exchange rich text format check box, and then click OK. 

Use the following steps to set plain text in the recipient's              contact record:              

1. Open the recipient's record in the Contacts folder.                         
2. Double-click the recipient's e-mail address.  
3. In the E-Mail Properties dialog box, click Send Plain Text only under Internet Format 

http://support.microsoft.com/kb/278061