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Basically, it happens when Outlook combines the attachment within the mail message as Rich Text. Any other client other than MS Outlook will see this as Winmail.dat. Some times I also face this problem when I check email on MDaemon World Client(web based). <br />
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Check the following link on MS KB, <br />
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<a href="http://support.microsoft.com/default.aspx?scid=kb;en-us;830302" rel="ugc nofollow">http://support.microsoft.com/default.aspx?scid=kb;en-us;830302</a><br />

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On sending mail with PDF attachment from Outlook 2007 using pop3 account, the mail is received as a DAT file to the recepients using outlook express, thunderbird and Oulook 2000. 
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On sending mail with PDF attachment from Outlook 2007 using pop3 account, the mail is received as a DAT file to the recepients using outlook express, thunderbird and Oulook 2000. 

Basically, it happens when Outlook combines the attachment within the mail message as Rich Text. Any other client other than MS Outlook will see this as Winmail.dat. Some times I also face this problem when I check email on MDaemon World Client(web based). 

Check the following link on MS KB, 

http://support.microsoft.com/default.aspx?scid=kb;en-us;830302


PDF attachment received as a DAT file in outlook express, thunderbird and outlook 2000 when sent from Outlook 2007 using pop3

Microsoft Outlook is a personal information manager from Microsoft, available as a part of the Microsoft Office suite. Although often used mainly as an email application, it also includes a calendar, task manager, contact manager, note-taker, journal, and web browser.

Outlook

Thunderbird is an email, newsgroup, news feed, and chat (XMPP, IRC, Twitter) client that supports POP and IMAP and LDAP address completion. The vanilla version is not a personal information manager, although the Mozilla Lightning extension adds PIM functionality. Thunderbird can manage multiple email, newsgroup, and news feed accounts and supports multiple identities within accounts. Features such as quick search, saved search folders, advanced message filtering, message grouping, and labels help manage and find messages. Thunderbird incorporates a Bayesian spam filter, a whitelist based on the included address book, and can also understand classifications by server-based filters such as SpamAssassin. Extensions allow the addition of features.

Thunderbird

Windows Live Mail is the successor to Outlook Express, a discontinued email and news client bundled with several versions of Microsoft Windows, from Windows 98 to Windows Server 2003, and available for Windows 95 and Mac OS 9. In Windows Vista, Outlook Express was superseded by Windows Mail, then again by Windows Live Mail as separate software. Outlook Express is a different application from Microsoft Outlook. Beginning with Windows Vista Microsoft included Windows Mail, based on large parts of Outlook Express source code.