I have had the problem of no being able to send an email to a certain few domains. No smtp log instance is ever created. The email just stays in the mail.box until it expires. The following are the things I had to do to fix this. Note that I ahd done all of these in the past, but after I made sure they were all done at the same time did I see successful results.
Verified MTU, using Ping -f -l 1472 mailserverIPaddress
changed the packet size, anything over 1472 would fragment
In Notes.ini, set DNSServer=externalDNS,InternalDNS
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces
I deleted the extra interfaces (the ones that were blank or incorrect)
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\nameserver
I set the nameserver to InternalDNS
Restart Task Router
I have tried all of these before, however I think they were never done at the same time