Email clients are conservative. Custom fonts load inconsistently, so a unicode headline is often the only styled text that survives Outlook, Gmail and Apple Mail at the same time.
Write the headline first, then run the key phrase through Font Style Generator. Paste the styled version above the plain-text body and keep the plain version in the alt text and the subject line.
Test the send to yourself on mobile before the real one; character fallbacks show up there first.
Written by FontStyle - more at fontstyle.io.