Outlook has a problem, When connecting to Rackspace?

Question

I'm working on a problem with Outlook O365 where, after a short while of working, mail would suddenly become stuck or take a while to transmit. We have two cloud exchange servers (totally separated and different domain), one hosted in Rackspace and the other by Microsoft. We have performed multiple isolation on WAN/Lan and even replaced components like the firewall, but Outlook is still having problems, and it appears that we have exhausted all network-side restoration options.

Notably, there is no indication of packet loss, from the networking device, we can see that every packet is transmitted to Rackspace and the client, but when we run a wireshark on the client, it appears that "tcp previous segment not captured" events are happening. Is there any reason why? it possible due to the target destination?

See the Topology, The only problem is when clients at small offices connect to O365 Rackspace; however, when connecting to Microsoft directly there is no problem. Another thing, from the main office connection on both connection to Microsoft and Rackspace cloud Works? So kind of hard to pin point the issue? Additionally, the only left item to investigate is what is happening at the Rackspace end to see what is going on between clients in the Small office and Main office.

If you can view the traffic flow in the topology, you will notice that there is a difference between the traffic flow via the small office and the main office.

Topology: https://ibb.co/z4SKFzR

Issue(small office to rackspace): https://ibb.co/qgLPhy0

Last but not least, if you notice the real problem, mail getting stuck, yet the status still indicates that the client is connected to Microsoft Exchange.

Please share your ideas or any actions that needs to do.

Thank you,