Fix: "winmail.dat cannot be opened"
You got a winmail.dat file because the sender's Outlook sent the message in TNEF format — a proprietary Microsoft packaging format — instead of standard MIME. Your email client doesn't understand TNEF, so instead of seeing the real attachments, you just see this opaque winmail.dat file.
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About .pub to PDF conversion
What is actually inside winmail.dat and how do I get to it?
winmail.dat is a container created by Microsoft Outlook that wraps the message body, formatting, and any real attachments (often .pub, .docx, .pdf, or other files) into a single TNEF-encoded blob. To extract the contents, use a free tool like Winmail Opener (Mac), winmaildat.com (web-based), or TNEF's Enough (Linux/Mac). Once extracted, you'll have the actual files the sender meant to attach.
The attachment inside winmail.dat is a .pub file — can OpenAny help with that?
Yes. Once you've extracted the .pub file from winmail.dat using a TNEF tool, you can upload it to OpenAny and convert it to PDF for free — no account required and your file is deleted within 60 seconds. OpenAny cannot open winmail.dat itself; you need to extract the .pub file first.
Can I ask the sender to fix this so it doesn't happen again?
Yes, and it's a one-time change on their end. In Outlook, they should go to File → Options → Mail and set 'Compose messages in this format' to HTML or Plain Text rather than Rich Text. Rich Text is the setting that triggers TNEF encoding for external recipients, so switching away from it stops winmail.dat from being generated.