Why do I see a thin 1px line between sections in Outlook for Windows?
You may occasionally notice a very thin horizontal line between sections of your email when viewed in Microsoft Outlook for Windows. These lines often look like a 1px border, even though no border or divider has been added in your Poppulo design.
This is a known rendering behavior in Outlook for Windows and is not caused by your email content or by Poppulo.
Why this happens
Unlike most modern email clients such as Gmail, Apple Mail, Outlook for Mac, or mobile apps, Outlook for Windows uses the Microsoft Word rendering engine to display HTML emails rather than a web browser engine.
Because Word was not designed to render responsive HTML email layouts, it has long-standing limitations in how it handles:
- Tables and nested table structures
- Background colors
- Borders and spacing
- Image scaling
- Fractional pixel rounding
As a result, Outlook may sometimes display a thin “hairline” between sections, even when:
- No border has been added
- No divider exists in the HTML
- The design appears correctly in all other email clients
The line is introduced during Outlook’s rendering process and does not exist in the original email code.
Why it appears inconsistent
You may find that:
- Some recipients see the line while others do not
- The line appears and disappears when zooming in or out
- The issue only affects specific machines
This is because the behavior can be influenced by:
- Windows display scaling or DPI settings (for example 125% or 150%)
- Outlook zoom level (for example 90% or 110%)
- Font rendering differences
- How Outlook rounds fractional pixel values when scaling content
- Minor differences in Outlook build or version
- Content that has been copied and pasted into the email
Even small variations in these settings can cause Outlook to draw a 1px gap between table rows or colored sections.
Is this something Poppulo can fix?
Because the line is generated by Outlook during rendering, it is not something intentionally added to the email and is not something Poppulo can directly control or prevent.
The same email will typically render correctly in:
- Gmail
- Apple Mail
- Outlook for Mac
- Mobile email apps
- Web-based email clients
This behavior has existed in Outlook for Windows for many years and is widely documented across the email industry.
Will this still happen in the new version of Outlook?
Microsoft’s new Outlook for Windows uses a modern, web-based rendering engine rather than the legacy Microsoft Word engine.
As a result, once recipients move to the new version of Outlook, they should no longer experience this 1px rendering artefact.
If you are seeing this issue and would like us to review your email, please contact Poppulo Support with a screenshot and details of the Outlook version and Windows scaling settings being used.
