Fix: Your Email Signature Shows as an Attachment in Outlook (ATT00001, image files, winmail.dat)
You send a nicely designed signature and the person on Outlook replies asking about the “attachment” you sent — or your logo shows up as ATT00001.png or a winmail.dat file. It's one of the most common and most embarrassing email-signature failures. Here's why it happens and how to stop it.
Step by step
Understand the root cause
Almost always: the image in your signature is embedded/attached rather than referenced as a hosted web image. Mail clients attach the raw image data, and the recipient's Outlook lists it as a file (ATT00001.png/.htm) instead of showing it inline.
Know the winmail.dat variant
If recipients get a winmail.dat file, that's the sender's mail being sent in Microsoft's proprietary RTF/TNEF format. It's a sending-format problem (more common when the sender uses Outlook); the fix is sending in HTML, not Rich Text.
Stop dragging images into the signature box
Dragging or pasting an image into Apple Mail's (or Outlook's) signature editor embeds the image data. That's exactly what produces the ATT00001 attachment on the other end. The image needs a hosted https URL instead.
Switch to hosted images
A signature whose images live at stable https URLs renders inline everywhere and never arrives as an attachment. This is the core fix — and it's what Autograph does automatically for every image you add (more on broken images).
If you send from Outlook, send as HTML
For the winmail.dat case: set your outgoing format to HTML (not Rich Text), and for Outlook on Windows, disable TNEF. Apple Mail sends HTML by default, so Apple Mail users rarely hit this.
Rebuild the signature cleanly
Recreate the signature with hosted images rather than trying to patch the attached one — a half-fixed signature often still carries the embedded image. Design it in Autograph and install it fresh into Apple Mail.
Verify with an Outlook recipient
Send a test to someone on Outlook (or a free Outlook.com account) and confirm the image shows inline with no attachment listed. That's the only test that counts.
Never send a “signature attachment” again
Autograph hosts your images and builds email-safe HTML, so your photo and logo render inline everywhere — no ATT00001, no winmail.dat. Install it into Apple Mail in one click.
Download Autograph free See the templatesCommon questions
Why does my email signature show up as ATT00001.htm or an image attachment?
Because the image is embedded in the message rather than hosted online. The recipient's client — often Outlook — lists the embedded data as an attachment instead of rendering it inline. Switching to hosted https images fixes it.
What is the winmail.dat file my recipients get?
It's Microsoft's TNEF/Rich Text format leaking through. It happens when mail is sent as Rich Text rather than HTML (usually from Outlook). Send in HTML format and disable TNEF to stop it.
How does Autograph avoid this?
Autograph hosts every image at a stable https URL and builds email-safe, table-based HTML — so signatures render inline in Apple Mail, Gmail, and Outlook and never arrive as attachments.