My Email Settings
Each Salesforce User can set personal settings related to emails they may send to contacts directly from a contact record.
Email Settings in Salesforce Lightning
Click on the icon on the upper right which is your personal settings. Click on the "Settings" link to customize your settings.
select "My Email Settings" from the many options available to you in personal settings.
Here you can set your email name, email address (for replies), and add a Signature. These settings will apply to any emails you send directly from Salesforce to a contact. (Note: this does not affect automated emails that are sent via workflows or processes)
In Salesforce Classic:
Under your 'name' menu in the header, select "My Settings"
Select My Email Settings from the left menu: