Different line heights in Outlook (old vs. new)

Problem

Email signatures are displayed differently in Outlook:
Line spacing appears noticeably different between the old and the new Outlook versions.

Cause

This is due to the different rendering technologies used by the two Outlook versions:

New Outlook

• based on modern web technology
• uses standard CSS
• line-height is rendered exactly as defined (e.g. 1.2, 1.5, etc.)

Old Outlook (classic)

• uses the Microsoft Word rendering engine
• does not support true CSS line-height
• instead applies its own predefined line spacing values, such as:
  • single
  • 1.5 lines
  • double

What does this mean in practice?

If a custom line height is defined in the signature (e.g. line-height: 1.2 or 1.5):

New Outlook
→ displays the spacing correctly

Old Outlook
→ ignores the defined value
→ applies its own default spacing (usually “single” ≈ 1)

Result:
The signature appears more compact or differently proportioned in old Outlook.

Solution / Recommendation

If the signature should look as similar as possible across both Outlook versions:

→ Set the line height consistently to 1

Why does this work?

• A value of 1 most closely matches the “single” spacing used in classic Outlook
• This minimizes visual differences between both versions

Important:
A 100% identical appearance is technically not possible due to the fundamentally different rendering engines.