WCF / ASMX :: System.Net.WebException - "The Operation Has Timed Out"

I know this has been asked umpteen times but i cant find an answer to my problem. Very simply, i have a webservice client which is trying to call a webservice. The webservice may take any time from 5mins to 30mins (or more) for downloading message data from pop3 server . However after 2mins i get the following error:

System.Net.WebException: The operation has timed-out

This is an issue with the webservice client timing out calling the webservice - as the underlying webservice still runs in the background
Sequence osf steps when the error occurs:

1.User logins with his credentials
2.Webservice client calls webservice to connect to pop3 server and download messages for that particular user.

Code in webservice for downloading data from pop3 server where the exception occurs

private string ReadOutput(bool bMultiline)
{
try
{
StreamReader srReader = new StreamReader(conn);
string strRet = srReader.ReadLine();
if (bMultiline)
{
// Example #4: Append new text to an existing file
string strLine = "";
while (true)
{
// In a multiline message the . indicates end of message
if (strLine == ".")
break;
// Relay will convert any line with just a . in it to ..
if (strLine == "..")
strLine = ".";
strRet += "
" + strLine;
}
}
return strRet;
}
catch (Exception e)
{
return null;
}
}

3.Exception is caught in webservice client while webservice continues to run.This exception occurs only when the user receives a large message may be above 1mb because i tested with 1MB msg file where i dint have any problem receiving or downloading from
pop3 server where as with 5 MB message file i got this exception .

After going through couple of forums i tested my luck by changing web.config in both webservice and webservice client where i set the attribute of httpruntime Execution timeout to 300 seconds with no success <httpRuntime

executionTimeout="300"></httpRuntime>.

I have this webservice client and webservice running on localmachine(OS-Windows Vista)