Is line-height ruining your day? Try this!
I haven't trusted line-height for years just because of friends like Outlook that doesn't just care that much about copy line spacing. But I was asked hundreds if not thousands of times to have the spacing as per the creative PSD file. Oh, the account people and the clients could be sometimes very stubborn!
What did I do? I kept looking into it until one day when I replaced em with px. Guess what? It WORKED!
So here is the fix for you:
<span style="border-collapse: collapse; font-family: 'Arial Black', Helvetica, sans-serif; font-size: 24px; font-size: 24px; font-weight: 900; color: #000000; line-height: 27px; mso-line-height-rule: exactly" class="Arial_Black">Your super cool message here!</span>
Notice also mso-line-height-rule: exactly to go with line-height. Awesomeness!
Using pixel to define the line height instead of em will also help having a more consistent line spacing across all email clients and devices.
Happy email coding! Your friend, the email snob.