Hi,
Please try these steps:
Select your vCenter Server and go to Configure menu
Under General, provide your Mail server (e.g mail.smtpserver.com) and mail sender (e.g vcenter-notification@smtpserver.com)
Once done, go to Advance Settings and look for
mail.smtp.password and provide the value by clicking the Edit
same with mail.smtp.port and set port 25
then lastly, provide the mail.smtp.username
Once done click Ok
Good Luck!