421 4.4.2. connection dropped error on some domains:
BDAT is part of ESMTP protocol – while Ex2007 supports ESMTP, it does not support BDAT due to vulnerabilities that would crash smtp server (Exchange 2000 era). You can see all the ESMTP commands Exchange 2007 supports by telnet to the server and execute EHLO. While only very small number of domains still use BDAT, it is not necessary to make any changes to Exchange 2007.
If you want to fix this problem, you have to disable ESMTP on Exchange. To do this, issue this command on the HUB transport server:
Set-SendConnector -Identity <name of your send connector> -ForceHELO $true