When you send a fax from Microsoft Outlook, you may receive a delivery failure email that reads:
Your fax was not sent because the following file attached to your email is a file format our system does not accept: winmail.dat
This happens even when the files you attached were PDFs, Word documents or images that Fax.Plus fully supports.
Why this happens
Outlook can send messages in a Microsoft-only format called Rich Text Format (RTF, also known as TNEF). When it does, Outlook packs your formatting and all of your attachments into a single file named winmail.dat and sends that instead of the individual files.
Only Outlook and Exchange can open that file. Fax.Plus receives one attachment called winmail.dat, cannot read it, and cannot find the documents inside, so the fax is not sent.
Note: This is not caused by the files you attached or by your Fax.Plus account, which is why the error appears even when your attachments are ordinary PDFs. It also explains why the same message may send correctly one day and fail the next: the format is decided per message and per recipient.
How to fix it
The solution is to stop Outlook sending in Rich Text Format when it writes to fax.plus addresses. Choose whichever of the following fits your situation.
Fix one message you are about to send
- In the message window, open the Format Text tab.
- Select HTML or Plain Text instead of Rich Text.
- Reattach your documents and send the message.
Fix it for all outgoing messages
- In Outlook, go to File and then Options.
- Select Mail.
- Under Compose messages, set Compose messages in this format to HTML or Plain Text.
- Scroll down to Message format and set When sending messages in Rich Text format to Internet recipients to Convert to HTML format or Convert to Plain Text format.
- Click OK.
Note: Both settings matter. If only the first one is changed, replies to messages you received in Rich Text Format can still be sent as winmail.dat.
Fix it for one recipient permanently
Use this if the error continues for a specific fax number even after changing the settings above.
- Open Outlook and start a new message.
- In the To field, right-click the fax.plus address you send to.
- Select Open Outlook Properties.
- Find Internet Format and set it to Send Plain Text Only.
- Click OK to save.
Tip: If your whole organization sends faxes from Outlook, your IT administrator can disable Rich Text Format for the fax.plus domain on your mail server. This resolves the problem for every user at once, and is the best option if several colleagues are affected.
If the error continues
- Confirm the attachments are in a supported format: PDF, DOC, DOCX, XLS, XLSX, TXT, RTF, JPG, PNG or TIFF.
- Check that you attached the documents as files rather than embedding or pasting them into the body of the message.
- Send the fax once from the Fax.Plus web app to confirm the documents themselves are fine.
- If it still fails, contact us at support@alohi.com with the failure email you received and we will investigate.