There is no Connection Filtering set in the Message Delivery properties.
The Block List Service Configuration list is empty.
Also the Exception list is empty as are the Global Accept and Deny lists.
I tried switching off TLS and restarting the SMTP Virtual Server but that did not fix the problem.
I'll try and answer all these questions:
Can both of you compare the anti-spam, virus, and reputation services you are using on your exchange servers?
>> Unless this is a MS feature, it is...
At last I have found someone with excatly the same problem as me.
I also see the same problem whereby the 1&1 server starts an SMTP connection and goes through all of the commands except the DATA command.
I have spent quite some time on the phone to 1&1 support but all they tell me is that...