I recenly rebuilt a vCenter environment for a customer. We decided to use the vCenter Server Appliance 6.5. The configuration of the vCenter Server Appliance was fairly simple and operates very similar to vCenter Server installed on Windows. We attempted to setup email alerts, but were unable to get the alerts to send. We initially thought the alerts would not send due to an issue with the SMTP relay. Since this was not a Windows OS, I was not able to login to the OS and test the STMP relay using telnet. I checked my configuration of email alerts several times and the administrator of the SMTP servers checked his as well and everything looked correct on both sides, but emails still would not send.
After researching for quite some time, I found that I could use the "mailq" command to view the email queue on the vCenter Server Appliance. I connected to the vCenter Server Appliance via SSH, ran the "shell" command to get to the full shell, and then ran the "mailq" command. This showed me that several messages were in the mail queue and not being sent. I began to troubleshoot this more and eventually found an VMWare article regarding a bug in the vCenter Server Appliance 6.5 that prevented SMTP from working correctly. This article had been published one day before I found it, which was about a month after I first started troubleshooting the issue. From looking at the files, the original code had the wrong patch in the sendmail.cf file.
Here is a link to the VMWare article with instructions on how to fix the bug: https://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2148396
The following must be done to successfully SCP the file to the vCenter Server Appliance: https://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2107727