Outlook email signature line-height problems
Classic Outlook can behave differently from browser-based mail clients when line spacing is expressed with CSS that its Word-based HTML engine does not support in the same way.
What Microsoft documents
Microsoft’s current rendering troubleshooting guidance says that numeric line-height values such as 1.5 are not supported in Classic Outlook custom HTML and recommends a percentage value such as 150%. It also notes that applying line-height to span elements can contribute to visible white-line or spacing artifacts and recommends applying it to a paragraph element instead.
Patterns SigLint checks
<p style="line-height:1.5">Name</p>
<span style="line-height:150%">Role</span>The first pattern triggers the unitless line-height rule. The second triggers the span line-height rule. SigLint reports the exact occurrence and links it back to this guide.
Safer direction
<p style="line-height:150%">Name</p>Use a supported percentage value and apply line-height to the paragraph where practical. Then re-run SigLint and test the installed signature in the actual Outlook versions you support.