I have Linux Rehat 8 installed on my machine, i use a proxy server to browse.I successfully configured the proxy settings on the Mozilla browser. Now i can browse other websites, but i am unable to go to sites like mail.yahoo.com, gmail.com. I also configured other mail client applications and unable to check my emails. My firewall level is always high and unable to change it. I also flushed all the iptables rules, still unable to slove this problem. pls help.

can you provide us with a description of what exactly happens when you are "unable to go" to these sites?? like, are you seeing an "Access Denied" error being generated by the proxy server, for example?? if so, then you should check to make sure your ACLs allow the proxy to establish outbound connections to HTTPS (tcp port 443) sites...

Public proxy servers often get blocked by big service providers.

Using a proxy is debatable for online privacy really - your isp will still see the traffic unless you are connecting via a secure link and of course the proxy gets to see all the traffic as well so you have just added another person in the middle.

The reason for blocking could be varied - but I suspect they are doing it for your security - if you connect to gmail and supply a password the proxy folk have it.

If it is not a public proxy you are using then it could still be related, but less likely gmail could detect it.