routing from 2007 to 2003

we have 2 routing group connector in our mixed mode environment: a default one and a second one that I 've manually created. for some odd reason the default one started to build up queue and not deliver to the 2003 mailboxes. I saw an error '451 5.7.3 Cannot achieve Exchange Server authentication...' on the 2007 side. Exchange 2007 uses Certs/TLS while 2003 doens't so integrated wind0ws authentication is used to allow communication. I checked the permission setting on the access tab of the 2003 virtual smtp and it was so. for some reason la-exchhub02 couldn't connect to bh-evs01 where as other hub servers were doing this. the thing was hub transport is that if one is designate as source transport server, the other hub servers should be able deliver to 2003 mailbox servers by passing their mails to the designagted hub server. this was happening but some emails were just getting stuck on the routing group. this was resolved by editing the 'interop rgc's source transport server on the 2007 side. by using 'Set-RoutingGroupConnector -Identity "f958a918-6797-41af-a8ad-5fddb6225063" -SourceTransportServers "la-exchhub01","la-exchhub02"'

after this restarting the transport service cleared the queue