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Issue #710: Feb 28 – Mar 6, 2021

Q: I always enjoy your weekly articles and your annual Christmas light show. I have an email question: When I send a PDF attachment from my desktop computer operating with 2004 Win 10 Home and Office 365 2016/Outlook 2012 configured to send messages in HTML, some recipients see the PDF attachment as a win.dat file while others see the attachment as a PDF file. An example: I recently sent an email from my desktop computer from my @cox account with a PDF attachment to my Samsung S10e. On my smartphone email @cox account, the attachment was received as a win.dat – unusable, but on the same phone email @gmail account the attachment was desired PDF format. Is there a way I can send a PDF attachment that most likely others will be able to open?

– John L.
Niceville, Florida

A:  Thanks for all the compliments, John, especially on the Geek Lights on the Corner.  From the outside looking in, I’m sure everybody thinks that this time of the year is down time for Spouse Peripheral and me.  Nothing could be farther from the truth.  During the show season, people are always asking how long it takes to set things up, and I tell them we work on the show pretty much year-round.  As the third month of 2021 is about to begin, orders for new components are being placed, and stock is rolling in the door.  Lighted wire-frames (what we call our “critters”) are being repaired and refurbished. Trains are being repaired.  And new show elements are being designed.  Yes, this stuff really goes on all year long.  It’s a labor of love, but it’s always nice to hear that our efforts are appreciated.

Moving right along, the file Win.dat, or more commonly, WinMail.dat is something that’s automatically generated by Microsoft Outlook when you compose an e-mail in Rich Text Format (RTF), or in some configurations of Hypertext Markup Language (HTML) format.  Just to be sure I don’t use tech jargon in my response without fully explaining, RTF is a proprietary file encoding format developed by Microsoft for exchanging information between Microsoft products.  Naturally, there are non-Microsoft uses for such technology, and Microsoft publishes a specification to keep the world informed of how RTF encodes data.  HTML, of course, is the same universal standard used by every web browser to encode and transfer data.  Both of these formats allow your e-mail to contain more than mere text.  RTF and HTML both allow encoding of such formats as boldface, italics, typefaces, font sizes, colors, and more.

When you use Outlook, and you choose RTF or HTML as your e-mail format and compose an e-mail, an extra file – either Win.dat or WinMail.dat file is automatically generated.  If you attach a file to it, the attachment is encoded right into the supplemental file.  If you send such an e-mail to someone else who uses Outlook, they’ll never see the Winmail.dat file.  Outlook doesn’t display it, because it’s not something that’s intended for the eyes of a person reading the e-mail.  Outlook simply decodes it, and puts everything into the proper format before presenting it to the reader.  Unfortunately, Outlook is the only e-mail client that knows what to do with a Winmail.dat file.  For any other client, it appears as an attachment to the incoming e-mail.  In general, you can prevent this from happening by using either Plain Text or HTML encoding, but there are a few cases in which an HTML-encoded message will cause file attachments to be encapsulated within a Win.dat or WinMail.dat file.  I’m out of space to describe these here.  To learn more, hit the web and visit  That article contains much of what I’ve said here, plus information on setting Outlook message formats in HTML that won’t cause it to generate a Winmail.dat.

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