You send an email with a normal signature. The recipient replies asking what the mysterious image001.png attachment is — or, worse, gets a file called winmail.dat they cannot open.
This is one of the oldest quirks in Outlook, and it has three distinct causes. Here is how to tell them apart and fix each one.
Outlook can send a message in three formats: HTML, Rich Text (RTF) and Plain Text. Only HTML behaves the way the rest of the email world expects.
In Rich Text, Outlook packs all formatting and images into a proprietary container. Non-Outlook clients cannot read it, so they display it as a raw attachment. That attachment is winmail.dat.
Separately, even in HTML mode, Outlook embeds images as inline attachments with a content ID. Those images display correctly, but they also appear in the attachment list. That behaviour is normal and not a bug.
This is the serious one. To change the default in classic Outlook:
In new Outlook and Outlook on the web, Rich Text is not offered at all, so this cause does not apply.
If the default is already HTML but one particular message misbehaves, open that message, go to the Format Text tab on the ribbon, and click HTML. The format switch applies immediately to the message you are writing.
Worth knowing: replying to a Rich Text message makes Outlook reply in Rich Text. So the format can be inherited from the person who wrote to you.
Outlook stores a per-contact format preference, and it silently overrides everything else. If the problem happens with one recipient and nobody else, this is why.
In managed environments an administrator can set the outgoing message format through Group Policy, and your local setting will be reset every time Outlook starts.
If you change the format, restart, and find it back on Rich Text, this is your answer. It has to be fixed by whoever manages your tenant.
Even in HTML mode, an image pasted directly into an Outlook signature is stored as an embedded attachment. It works, but it makes every message heavier and produces the image001.png entries that confuse recipients.
Referencing images from a public HTTPS address instead keeps the message light and leaves the attachment list clean. It also means you can swap a banner across the whole team without anyone touching Outlook.
If images then fail to appear, work through our guide to email signature images not showing.
Send a message to a Gmail address and to a colleague on a phone. If both show the signature inline with no stray files, the format is correct.
A signature built in the Mail Ink generator is clean HTML with hosted images by design, which removes two of the three causes above before you start.