Outlook sent a PDF, but the recipient got winmail.dat

A sender on Microsoft Q&A asked whether the fix belonged on their computer or the recipient's. Only some people were getting an unreadable winmail.dat attachment. On checking the compose setting, the sender found it was already HTML. They switched to plain text but had not yet confirmed the result with the affected recipients. The January 2026 question and follow-up

Create a new HTML or plain-text message and attach the original document again. Ask the affected recipient whether that file opens. Classic and new Outlook for Windows put these format controls in different places. If the new message still delivers winmail.dat from a work account, the Exchange Online administrator can check the settings applied to that recipient and domain.

Getting the original document resent

If you are the recipient, changing the format of messages you compose will not unpack a file that has already arrived. Ask the sender for the original document in a newly composed message. Give them the name you expected and the name you actually received, rather than simply saying that the attachment is broken.

I received winmail.dat instead of the PDF. Could you create a new HTML or plain-text email and attach the original [document name] again? Please use the original file rather than attaching the winmail.dat file from the previous message. I'll confirm whether the document opens.

The sender can open the local original before attaching it, check the document title and revision, and send a clearly labelled replacement to the person who reported the problem. This avoids circulating an older version while trying to fix delivery. Start with one affected recipient instead of sending repeated replacements to the entire customer list.

Agree on what the reply should confirm: the expected filename is visible, the document opens, and its contents match the intended revision. A reply saying “received” may mean only that the email arrived. For a document containing several pages, ask the recipient to check that the required pages are present too.

What winmail.dat contains

Outlook can package formatting and attachments using Microsoft's Transport Neutral Encapsulation Format, or TNEF. A receiving application that understands it can display the original content. Another application may show the package itself as winmail.dat. Regular attachments can be inside that package; it is not simply a PDF with the wrong filename. Microsoft's description of TNEF

Changing the extension to .pdf leaves the package's contents unchanged. Putting the original PDF into a ZIP file before sending is not a dependable workaround either: the ZIP can itself be encapsulated. Mail provider Infomaniak documents that limitation and distinguishes extracting one attachment from correcting future messages.

A TNEF extraction tool may recover a document when a resend is unavailable. For business material, use an approved tool rather than uploading the package to an arbitrary conversion website. If the sender still has the original, requesting that file also lets you confirm which document and revision should have been delivered.

Recipient differences are not confined to people using one particular mail app. A 2020 Korean support question described an Outlook IMAP user seeing winmail.dat while others received the message normally, even after reinstalling the PC. Include the receiving account and application with the filename when reporting the problem.

The two settings to check in classic Outlook

In classic Outlook, open File → Options → Mail and set Compose messages in this format to HTML. For the replacement email itself, open a new message and choose Format Text → HTML, then attach the original. Use Pop Out if you are composing within the Reading Pane. Microsoft's version-specific compose instructions

Plain Text is another choice. It removes body formatting and inline pictures, but you can still attach files. A missing logo in the signature therefore has a different explanation from a missing PDF attachment. Read the replacement's body before sending, particularly if an image contained information the recipient needs.

Check the current message as well as the default. An existing reply or forward is not a fresh message, and its format can be changed independently. Starting a new message gives you a simple place to select the format and attach the original document again.

If just one address keeps failing, keep a record of that address and the message format used. Recipient-specific settings can be relevant. Do not erase the entire contact list to investigate one customer's delivery problem; the administrator will need the affected recipient and a recent example.

New Outlook is already using HTML

New Outlook has no RTF compose choice. Its default is under Settings → Mail → Compose and reply → Message format; select HTML or Plain text and save. For a single plain-text message, pop the compose window out and select Options → Switch to plain text.

If HTML is already selected, send the fresh attachment and check the result. If it still arrives as winmail.dat on a work account, give that message to the mail administrator along with the recipient's address and the selected format.

For a personal account, a fresh message sent from the provider's webmail can be a useful comparison. Keep the recipient and original document the same, and note which sending route produced each result. If only one route fails, include that difference in the request to the mail provider's support team.

When Exchange changes the delivery format

Exchange Online distinguishes the format used to write a message from the format used to transmit it. Its documentation says that HTML, plain text and RTF messages can all be sent using TNEF. Administrative settings can therefore matter even when the sender has selected HTML. Message composition and transmission in Exchange Online

A remote domain applies settings to messages sent to an external domain. Its settings take precedence over the sender's Outlook choices and over per-recipient settings configured by administrators. A customer-specific configuration can consequently produce a different result from mail sent to another organization. Remote-domain scope and precedence

An authorized administrator can inspect Mail flow → Remote domains in the Exchange admin center. With Rich Text set to Never, RTF messages go out as HTML or plain text. Check a specific domain's configuration as well as Default: a conflicting specific setting wins. Managing remote domains

Changing Default can affect other external destinations. Scope a change to the actual delivery requirement, including any Outlook features in use. Ordinary file attachments do not require TNEF, but features such as voting buttons can. A fix for sending a PDF should not silently change how unrelated voting messages are handled.

Check files emailed from the accounting app

Some documents are emailed from another application through Outlook. Intuit has a specific support article for QuickBooks Desktop PDFs arriving as winmail.dat. After fixing a manually composed message, check the normal application workflow as well.

For example, suppose a PDF attached in a new HTML email opens correctly, but the same document emailed from the accounting application still arrives as winmail.dat. Record the two subjects and send times separately. Keep the file and recipient constant so that support can compare the sending routes. A document without sensitive business details can serve as the initial check.

Once the recipient opens the expected document, verify its revision and required pages before sending the remaining business files. If the replacement still fails, preserve that message for support instead of repeatedly forwarding the old package.