I've bumped into an issue using the codes that I know for a check mark in HTML - ✓ ✔ ✓ ✔

They are now failing in all email clients I test with (gmail, outlook, outlook 365, even email apps on phones). Has anyone else encountered this? Up until today, we have had success using these codes. Resending old, successful emails results in the same, horrible question mark! They all look great in browsers.