Same problem, a little different.I am having the same trouble. We cannot send/receive mail with a small number of domains. We discovered that the trouble is related to a NAT statement on the router, - 'IP nat inside source 192.xxx.xxx.xxx 200.xxx....
Maybe you need to specify the bandwaidth in the configuration?"bandwidth 1536", or something.I'm not sure I saw this referred to in your initial message. I aplogize if I'm repeating something you've tried.bob kahler