Sending an Email using Organization Wide Address in Salesforce

In Salesforce, click Setup and search in Quick Find for Organization.  Click on the option for Organization-Wide Addresses.

Click Add and in the following screen you can designate a Display Name, specify the email address and decide if all or just selected profiles can use the address.  Based on the permissions here, it determines if a User can select and use such an address when composing email in a composer in Salesforce.  After saving there is a verification step Salesforce requires before the address is available for use.

If a User then navigates to a record, clicks the Send Email, Reply, Reply All or Forward options, and launches the email composer, there is a carat for a drop down menu that will display the organization-wide addresses available to the User launching the composer.