All my installs are standalone. No FortiGate at all.
I first Shutdown FortiClient using tray icon right click.
Checked for incoming e-mails. It worked in my case.
So I run FortiClient diagnostic tool on that system. That took about 5-10 minutes to complete.
I then re-run FortiClient doing nothing in configuration files and no rebooting the system.
Checked for incoming e-mails. It worked in my case (I just couldn' t understand what fixed the problem at all)
I then rebooted the system. No change on config files, again.
I checked if Outlook is ok. It was OK.
I then shutdown the computer, waited some minutes. Re-open it. Try to see if Outlook is working, yes it is working.
Several trials on reboot and shutdown/re-open, nothing unusual now.
So, I am not sure, only logical explanation I can think of is FortiClient Diagnostic Tool is automatically fixing some very basic errors in config files.
That computer has no problem now.
Btw, I checked registry for key given above by ducachris, no such key was present in that system. Not even close.