Certificates / Synch HUB and Edge

[Goldenberg]

Exchange 2007 with SP2 installed on W2k3 - All roles on same server. All
updates are applied.

How can i assure that there is proper synchronization between HUB and Edge?

How can i assure that i have proper certificates installed? Is it possible
to have a conflict between certificates?

Frankly, i have done extensive troubleshooting for the errors "451 4.4.0
Primary target IP address responded with : 421 4.4.2 Connection Dropped,
attemted failover to....etc... "in vain"

I have assured that i have properly conigured DNS, proper nslookup, properly
opened ports. Properly configured MX records...

Emails to hotmail, gmail goes out and in normally but many other domains
responds in the queue with the mentioned errors above... the issue that after
certain retries, the messges goes out... but i am having plenty of retry
sessions...

Any idea?

Thanks,

--
Regards
Walid Natour