Go to Jira Administration and open the System Tab;
Select Customer notifications and click on “Configure new SMTP mail server”;
Add SMTP Mail Server. Provide its name and description and outgoing email address. Customize mail subject by adding email prefix if needed;
Configure the SMTP host. Choose your Service Provider, enter mail credentials;
Try Test Connection, and you’re done.
Visit Configuring Jira's SMTP mail server to send notifications to learn more details.