If you want to set the receive connector to accept only FQDN Helo from internet.
Type the following command in Exchange Management Shell to force the remote server to provide a domain name in EHLO handshake:
Set-ReceiveConnector –Identity "Default Connector" –RequireEHLODomain $True
The RequireEHLODomain parameter specifies whether the remote computer must provide a domain name in the EHLO handshake after the SMTP connection is established. Valid values for this parameter are $true or $false. The default value is $false. When the RequireEHLODomain parameter is set to $true, the remote computer must provide a domain name in the EHLO handshake after the SMTP connection is established. If the remote computer doesn't provide the domain name, the SMTP connection is closed.
You can set also the following useful parameters (Thanks to ObiWan)
ConnectionInactivityTimeout 1 or 2 minutes
For further informatin about it, please refer to the article below: