Thin horizontal lines sometimes appear in emails sent through Outlook. This is an Outlook rendering issue that affects classic and legacy versions of the email client.
Quick Solutions
Try these fixes to reduce or eliminate horizontal lines in your emails:
Match Background Colors
Set your email's background color to match the content area background color. This minimizes the visibility of horizontal lines.
- Open your email draft in the Email Builder
- Click the Settings tab on the right
- Set the Background color and Content area background color to the same value
Use Even Number Sizing
Outlook sometimes renders odd-numbered pixel values incorrectly, creating thin lines.
Change all measurements to even numbers:
- Font sizes: Use 14px instead of 13px or 15px
- Image dimensions: Use 200px × 200px instead of 205px × 205px
- Table heights and padding: Use 20px instead of 19px
- Line heights and borders: Keep all values even
Image resizing tools:
- Windows: Microsoft Photo Editor
- Mac: Photos app
- Cross-platform: GIMP (free)
Add Empty Rows
Adding an empty row between content blocks can help balance pixel rendering.
1. Open your email draft in the Email Builder
4. Click the Rows tab on the right
5. Drag a new row into your email
6. Leave the row empty (no content blocks)
Break Content Into Smaller Blocks
Large content blocks are more likely to display horizontal lines. Break your content into separate rows for each element.
Best practice: Use one content block per row. This also improves mobile responsiveness.
When Lines Appear
Horizontal lines are more common in these situations:
Recipient's Outlook version: Lines appear most frequently in Classic Outlook versions for Windows desktop. Recipients using New Outlook see fewer or no lines.
Zoomed-in views: Lines become visible when recipients zoom in on emails, as 1px spacing is relative to device view.
Misaligned content blocks: Content blocks of varying sizes that don't align well can cause lines to appear.
Stacked images: Outlook sometimes adds borders around images. Set all image borders to 0px to prevent this.
Forwarded emails: Original emails may look fine, but forwarding can introduce lines. This happens because forwarded emails are sent outside ContactMonkey through Outlook, and the HTML gets reformatted.
Why This Happens
Outlook's HTML rendering engine converts pixel measurements to points. When this conversion results in decimal values, Outlook adds the leftover fraction as a thin line. Microsoft has not officially addressed this long-standing issue.
Note: Recipients who enable the New Outlook client typically see fewer or no lines. However, both you and your recipients need New Outlook enabled for this to work. This option may be controlled by your IT department.
Test Before Sending
Send a test email to someone who previously saw horizontal lines in your emails. This verifies your fixes work before sending to your full recipient list.