Overview
You may notice thin white lines appearing across your email when viewed in Outlook for Windows. These lines can disrupt your design and layout.
This behavior is a known Outlook rendering issue. It cannot be fully removed. You can reduce how often it appears.
What Causes White Lines?
Outlook uses a different rendering system than most email clients.
Outlook is based on Microsoft Word
Word uses points (pt) instead of pixels (px)
1 pixel converts to 0.75 points
This conversion creates fractional values. These values do not align cleanly on screen. This results in visible gaps or “hairline” white lines.
When Does This Happen?
You are more likely to see white lines when:
Sections have tight spacing
Padding uses uneven numbers
Images have odd dimensions
Multiple modules stack closely together
How to Reduce White Lines
You cannot eliminate the issue. You can reduce its impact by adjusting your design.
Use Even Pixel Values
Use whole, even numbers for spacing.
Use
20pxinstead of21pxAvoid decimal values like
30.5px
This helps reduce rounding issues.
Adjust Padding and Spacing
Add extra space where lines appear.
Increase top or bottom padding
Add a line break in problem areas
Small spacing changes can remove visible gaps.
Use Even Image Dimensions
Set image sizes using even numbers.
Example: 600px × 400px
Avoid odd widths or heights
This improves rendering consistency.
Reduce Section Breaks
Combine content into fewer sections when possible.
Fewer sections reduce structural gaps
This lowers the chance of white lines appearing
Best Practices Summary
Recommendation | Why It Helps |
Use even pixel values | Reduces rounding errors |
Increase padding | Covers visual gaps |
Use even image sizes | Improves scaling |
Combine sections | Limits rendering breaks |
Future Outlook Improvements
Microsoft is updating Outlook to use a modern rendering engine based on Outlook.com.
This update will remove this issue over time.
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