I'm not sure how the web interface behaves with search-head clustering in this case but the email settings are configured in alert_actions.conf in [email] stanza.
Of course since you have clustered search-head environment, the settings are most probably deployed via an app deployed from the deployer. You should probably look there.
You can also just log on onto one of your search-heads and run
/opt/splunk/bin/splunk btool alert_actions list email
To see effective config. You can also add --debug switch to see which file the setting comes from.
You're interested in the "mailserver" key