The Email client icon is not displaying on the task bar even after adding the Email Client Attribute.
Release or Environment
There can be multiple causes for this issue. One such cause is being on UI16, where the email client icon was moved into the "more options" menu. Another cause could be a client script, which takes the email icon out of the "more options" menu and places it back on the form header on UI16, not being configured properly to run on the table in question.
To resolve the issue, switch to UI15, or make sure the client script is set to run on the specific table where the email client is not showing on the task bar.