Auction site eBay is investigating how its email icon was swapped for an image of a semi-naked woman.
The image appeared at the top of eBay UK messages viewed by users of Microsoft's Outlook mobile app.
The icon popped up on messages containing receipts, notifications and status reports.
An update applied to the Outlook app soon after the flaw was noticed returned eBay's icon to its familiar letter-based logo.
News about the icon swap circulated first on Twitter and on a forum called CasualUK on Reddit. The thumbnail image shows a young woman posing in front of a window wearing only underwear.
The news prompted speculation about how it had happened. One Reddit user suggested the image had been grabbed by accident from a computer used by admin staff who maintained the Outlook app.
The image may have been inadvertently grabbed as the app was undergoing maintenance.
Reddit and Twitter users also shared tips on how to update the app and get rid of the inappropriate icon.
Auction site eBay told tech news site The Register it was "investigating" the incident.
It said: "We are in touch with Microsoft - one of our email service providers in the UK - who is actively investigating the root cause of the matter."
EBay said there was no reason to think any user data or its systems had been compromised by the change.
Microsoft said it was also investigating how the icon had been swapped.
An update pushed out to the Outlook app over the weekend put eBay's logo back at the top of messages.