Restrict Use of Salesforce Classic HTML-Based Email Templates to Secure Browsers (Critical Update, Enforced)
Where: This change applies to Salesforce Classic in all editions, except Personal Edition.
When: This critical update is enforced in the Winter ’20 release.
How: Although we recommend not using Internet Explorer for HTML-based templates, you can override this restriction. From Setup, enter Session Settings in the Quick Find box, then select Session Settings. Select Override Restriction for Accessing HTML-Based Email Templates in Salesforce Classic Using Internet Explorer.