Send Email button is not in the Activity History related list in Classic nor Chatter actions in Lightning

The 'Send Email' button in the Activity History related list (Classic) and in Chatter Actions (Lightning Experience) only appears when the organization's email access level is set to 'All email'. If the access level is set to 'No Access' or 'System Email Only', the button is suppressed system-wide, even if it has been added to the page layout. The following steps resolve this issue.

Set Access Level to 'All email'

1. Navigate to Deliverability from Setup:
   1. In Salesforce Classic: Setup > Administer > Email Administration > Deliverability
   2. In Lightning Experience: Setup > Email > Deliverability
2. Under the 'Access to Send Email (All Email Services)' section, on the 'Access level' field, select All email.

After changing this setting, the 'Send Email' button becomes available from the 'Activity History' related list in Classic or as a Chatter action in Lightning.

Notes

• If the 'Access to Send Email' level is set to 'No Access' or 'System Email Only', and a user attempts to drag the Email button to Salesforce Mobile and Lightning Experience Actions in the page layout, the setting does not persist after saving. Setting the access level to 'All email' in Lightning resolves this layout issue.
• The 'Send Email' button is available in the Activities component (Accounts, Contacts, Contracts, Leads, Opportunities, and activity-enabled custom object records) and the Chatter component (Cases) in Lightning Experience.
• Users cannot send emails associated with Cases from the Global Actions menu in Lightning Experience.