As we know the importance of securing the web applications which are published on the internet have been increased.
So usually these external url’s are published in a secure way via reverse proxy which will handle this job.

When a end user logs into the OWA URL the session proxies via the published reverse proxy.
From Exchange 2013 we can notice that clicking on logoff will not trigger GET/OWA/Logff.owa like it was till Exchange 2010 where it generates a logoff page owa/auth/logoff.aspx?Cmd=logoff&src=exch
This log off page in 2010 was used by few Reverse Proxy to terminate the connection. This value can be modified in Exchange 2013 web.config file to bring this same page like 2010.

On Exchange 2016 we need to perform the below operation :

Navigate only to the below location

%ExchangeInstallPath%\ClientAccess\OWA\web.config

Remove the following line and do iisreset(make sure you make a backup of web.config before you do this):
<!– Disable logout page temporarily until UX is updated –>
<add key=”LogonSettings.SignOutKind” value=”LegacyLogOff” />

After performing this action the cookie session can be terminated.

Thanks 
Sathish Veerapandian

MVP – Exchange Server