For me it's
/usr/share/icons/ubuntu-mono-dark/status/24/applications-email-panel
or
/usr/share/icons/ubuntu-mono-dark/status/12/applications-email-panel
For me it's
/usr/share/ icons/ubuntu- mono-dark/ status/ 24/applications -email- panel
or
/usr/share/ icons/ubuntu- mono-dark/ status/ 12/applications -email- panel