You need to update the hostname setting under alert_actions.conf (If you don't have a local alert_actions.conf, create one and add the stanza & hostname setting below)
Should look like this:
[default]
hostname=your_hostname_here
Here is the link to the relevant docs
http://docs.splunk.com/Documentation/Splunk/latest/Admin/Alertactionsconf?utm_source=answers&utm_medium=in-answer&utm_term=alert_actions.conf&utm_campaign=refdoc
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