Four or five users (out of 30-35 users) at one of our office locations are having odd issues with Outlook 2010 and 2013 losing the connection to Office 365: Occasionally, when sending an email, the progress bar goes almost all the way to completed but then stays there.
If I open "Connection Status," the connections are all established but the Req/Fail counter doesn't increase. But if I open the connection monitor on my firewall, I can see TCP traffic to the Microsoft server slowly trickle across.
Cancelling the server request immediately fixes the issue (as will restarting Outlook) but only for that Send/Receive.
I'm using Cisco switches with a SonicWall TZ205 firewall. I was hoping the issue was caused by our firewall killing inactive RPC tunnels, but upping the TCP timeout to two hours has had no effect.
Microsoft hasn't been any help and they've stopped returning our phone calls.
If it is a local network issue, what would I even look for?
Try disable IPV6 on the host or the Router.
https://support.microsoft.com/en-us/kb/929852