I was seeing this in VCSA 6.7 and might apply in 6.5.  I found that all I had to do was to remove the smtp server from the mail server settings, save it, and then re-add the smtp server and save it again.  No service restarts or server restart required.  Seemed to stop working after an upgrade of the VCSA.  This tipped me off: https://www.ictstuff.info/vsphere-6-7-no-e-mail-notifications-from-vcenter/ .