Why flexbox is risky in Classic Outlook signatures
Classic Outlook uses a Word-based HTML processing engine rather than a modern browser rendering engine. SigLint therefore treats display:flex as a conservative layout-risk heuristic when a signature needs dependable Classic Outlook support.
Important evidence distinction: SigLint does not present this as a Microsoft-published “Flexbox rule.” Microsoft documents the underlying Classic Outlook rendering model; the Flexbox finding is SigLint’s conservative implementation guidance.
Recommended approach
For critical two-column signature layouts, use simple table markup as the compatibility baseline. Keep essential spacing on table cells, use explicit image dimensions, and test the finished signature in the actual Outlook versions you support.
What the checker gives you
SigLint reports every detected Flexbox occurrence, shows its line and column, lets you jump back to the HTML, and links the finding to this guide. Re-run the checker after replacing the risky layout to see what was resolved.