View Standard Fix
USE CASE Target

Fix CSS Flexbox Collapse in Mozilla Thunderbird — SendGrid Guide for B2B Cold Outreach

B2B cold outreach templates demand maximum deliverability and a plain-text appearance. Heavy, broken CSS structures or HTML syntax issues trigger spam filters instantly. Below is the targeted integration guide to fix CSS Flexbox Collapse inside Mozilla Thunderbird templates when using SendGrid.

Technical Breakdown

Mozilla Thunderbird uses the Gecko which lacks support for standard rendering rules. This causes CSS Flexbox Collapse elements to break or collapse when rendering templates.

SendGrid Behaviors

SendGrid's template parser automatically injects custom style tags that can override inline styles. Always use inline style definitions and define clear target widths. For cold sales outreach, minimize VML structures and prioritize light, standard text blocks to maximize inbox placement on corporate enterprise filters.

Remediation Code Block

To resolve this, modern layouts must be compiled down to Outlook-compliant nested HTML table models. Each flex container becomes a table row, and each flex child becomes a table data cell (td) with explicit pixel or percentage widths. In your SendGrid campaign manager, make sure you configure your links to run via {{{unsubscribe}}}.

<table role="presentation" border="0" cellpadding="0" cellspacing="0" width="100%">
  <tr>
    <td width="50%" valign="top" style="padding: 20px;">Column 1</td>
    <td width="50%" valign="top" style="padding: 20px;">Column 2</td>
  </tr>
</table>

Live Code Sandbox Test

Interactive SendGrid Custom Sandbox

Target Environment: Mozilla Thunderbird

How to Deploy inside SendGrid

1

Copy the fixed VML or table wrapper code block from the remediation box above.

2

Open SendGrid Design Editor, toggle the view mode to Code Editor, or insert a custom 'Code' module block inside your drag-and-drop layout. Drop the corrected code inside the box.

Targeted Fixes by Use Case & Industry