vCenter 6.5 can't send E-Mail notification

Hi Community,

my new vCenter Server 6.5 can't send notification E-Mails.

I have add the SMTP-Server (on the old vCenter 6.0 works without problems...)

I saw the Error-Message in the Log-File:

[VpxdMail] Failed to send the mail to SMTP server 192.168.0.99 at port 25. Error=Unknown com error

All other Parts from the vCenter works...

have someone a idea whats happen?

The vCenter-Server is a Windows 2016 Core installation.

thanks Lars