This has been around since I at least 6.5 I think (and must have been reported countless times, just did not find it).
Am on VMUG, so I can't submit a "real" bug report. Just hoping someone from VMWare picks this up and repairs it. And also hoping it might help others.
Every time one does an update of vCenter, SMTP mails/mail alerts are no longer sent.
The fault is in a wrong update of the underlying submit.cf during the vCenter update.
A rather complicated workaround is mentioned here: https://kb.vmware.com/s/article/54375
The easier workaround is here (screen names are from 7.0, they might have been different in earlier versions, but the process is totally identical):
Mails will work again. Also, all outstanding mails will get sent immediately.
It worked for me