How to modify the From address from an Email client

This KB describes you on setting up FROM address and Label using email client without modifying the email accounts

• Create an email client with required inputs from the table "sys_email_client_template"
• From the template you can add Sender Configuration
• Values can be added in multiple ways as shown below

If you wanted to add the "From" globally for all outbound emails, you can modify the email from the SMTP account and for the label, you can update from the property "glide.email.username"with a required one. 

See also https://support.servicenow.com/kb?id=kb_article_view&sysparm_article=KB0815999