Originally Posted by ghall
What exactly is happening here? Should I be concerned? Is there any way to make the server happy in order to not receive these warnings?
This happend when sending part does not get along with your TLS.
If this comes randomly every now and then especially with senders on dynamic-ip, don't worry at all, they are spammer-bots.
When sending mailserver does not understand TLS and can't find SSL, it should send with no encryption. If not, there is not much you can do for it.
We found out that one sender trying to reach our mailserver with this error was Windows 2000 server with amateur-configured Exchange. They was complaining us because their customers in our network did not receive their important(?!) marketing mails. We did advise them to get professional help
Referense from Postfix-mailinglist: