Q: When I send an attached PDF, some recipients say they cannot open the attachment. I use O360 Outlook to send from a desktop with Windows 10. I sent a test email to myself to see how my Samsung S7 handled the attachment. My S7 using @gmail.com was fine – PDF opened without problem. My S7 using @cox.net reported having a winmail.dat file attached, which I could not open. Any ideas as to why the PDF was changed to winmail.dat?

– John L., Niceville

A: File Winmail.dat is Outlook’s way of preserving data when an email is sent in Rich Text Format (RTF). This works just fine when the recipient of the email is using client software that recognizes and interpret the formatting instructions contained within the Winmail.dat file. For clients that don’t, Winmail.dat is just so much gibberish. It’s just a binary data that no program can open or display.

The solution to this problem is pretty simple: stop sending email in RTF format to people with an email client that can’t interpret Winmail.dat. But, you might ask, how do I know who that is? That’s simple too: you can’t. Well, you can if they’ve complained to you that they aren’t receiving your attachments. In that case, you could simply change the format of email messages you send to those people by clicking on the “Tools” menu, selecting “Options,” then clicking “Mail Format”. Under “Compose in this message format” select “Plain Text.” This will force Outlook to send the attachment without encoding it in Winmail.dat. Unfortunately, using Plain Text also means that you lose the ability to do any formatting in the body of your message. That is to say, you can’t choose your own type face, or font size, or even apply formats like bold, italic, or underline. It is what it says: plain text.

Unless there is a compelling reason for you to use RTF (and there probably is not in this day and age) a better alternative might be to switch your default email format from RTF to Hypertext Markup Language, or HTML. If that term looks familiar, it’s because you’ve probably seen it before – or at least a shortened version of it. HTML is the same encoding that web browsers use to do their thing, and many of the pages you view in your web browser have addresses that end in .html or, more commonly, ,htm. Unlike RTF, HTML doesn’t need a Winmail.dat file to preserve special formatting. And HTML is widely supported by almost every email client in use today, so you shouldn’t have any compatibility issues. Having said that, HTML is also used as a vector for the spreading of malware, so there are some clients, and some network administrators that don’t always like HTML-formatted messages. At worst, they may block the messages, or mark them as SPAM. More commonly, they strip-off the formatting, and display the message in plain text. The reader may or may not have the option to change the message back to HTML and view it as the sender intended.

To change Outlook to compose in HTML format by default, perform the following steps: In Outlook, click “File” and select “Options.” Go to the “Mail” category. Under “Compose messages” find “Compose messages in this format” and set it to HTML. If you don’t need or want to use any of the custom formatting that I mentioned above, you can also set this selection to “Plain Text.” Your emails will be simple, unformatted blocks of plain old (boring) text. But, your attachments will work properly, and you won’t ever see a Winmail.dat file again.

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