Hi Joseph TK Liu,
I want to confirm the following information with you:
1. How did you add DNS zone (a.com and b.com), please send the screenshot of them to v-rocwan@microsoft.com for analyze.
2. Did you subscribe your Edge Transport server to internal Active Directory site?
3. Open the Exchange Management Console on the EDGE server,
4. Select the EDGE server in the TOP, MIDDLE pane, and select properties, click ExternalDNS Lookup, send the screenshot of it to me.
5. How many NICs do you have on Edge transport server?
Exchange 2007 SP1 also includes a parameter in the EdgeTransport.exe.config application configuration file that is named DnsFaultTolerance. This parameter has the following values:
•Lenient (default setting)
•Normal
6. Send the EdgeTransport.exe.config file to me.
Related articles:
Configuring Transport Server Properties
http://technet.microsoft.com/en-us/library/aa997166.aspx
Send Connector Properties > Network Tab
http://technet.microsoft.com/en-us/library/bb690960.aspx
Rock Wang
Rock Wang– MSFT