Email alerts stopped after vCenter 6 to 6.5 upgrade

Upgraded out vCenter from 6.0 Appliance to 6.5 Appliance last week.
It was one of my smoothest experience with vCenters all these time. Having said that, after two days I realised that the email alerts stopped working. We had same issues when we upgraded 5.5 Windows to 6.0 Appliance and same thing happened. We had to go though a bunch of manual config changes to get it working below is the KB and worth checking
https://kb.vmware.com/s/article/2073849
I was afraid that it's the same case again, but to my surprise it was not the case here.

If you are experiencing slimier issues, follow below steps

1) Open vSphere Web Client
2) Click on vCenter and Click on Configure at the right side
3) Under Settings -> General Click Edit
4) Click on Mail
5) Cut the Mail Server (same in a notepad if required) and type something else there (may be check@email.com) and click OK to same
6) Edit again and replace the Mail server with the actual SMTP FQDN that you cut earlier.

Boom!!

All the emails started flowing to my inbox again and there were a lot of them. Looks like they were queued.

Give it a try... All the best....