Send emails as (Optional) Specify a sender identification, used in the From email header field. Use an email address or a string. Strings are concatenated with @<hostname>, using the hostname specified in alert_actions.conffor the machine sending the email notification or @localhost if no hostname is specified. Defaults to splunk@<hostname> or splunk@localhost if no hostname is specified.
Or via GUI, the base settings are
As an admin go to Settings -> Server settings -> Email settings -> Send emails as and set to proper value.