proxy authentication with {RCurl}

Being not an expert on web-programming, I attempt to automate a task which consists of getting into a website, download some csv files, and finally import them in R for data analysis.

In this context, I am working on the below sample code found on internet, which has been customized a bit to my need, and wonder to know more about the resulting error:

library(RCurl)
curl = getCurlHandle()
curlSetOpt(cookiejar = 'cookies.txt', followlocation = TRUE, autoreferer = 
TRUE, curl = curl)

# Load the page for the first time to capture VIEWSTATE:

html <- getURL('http://ift.tt/21avmm9',  
curl = curl, 
           .opts=list(ssl.verifypeer=FALSE))

# Extract VIEWSTATE with a regular expression or any other tool:

viewstate <- as.character(sub('.*id="__VIEWSTATE" value="([0-9a-zA-  
Z+/=]*).*', '\\1', html))

# Set the parameters as your username, password and the VIEWSTATE:

params <- list(
'user'         = '<USERNAME>',
'pass'         = '<PASSWORD>',
'__VIEWSTATE'                                  = viewstate
)

html = postForm('http://ift.tt/21avmm9',   
.params = params, curl = curl,.opts=list(ssl.verifypeer=FALSE))     
 Error: Proxy Authentication Required

# Verify if you are logged in:
grepl('Logout', html)
[1]FALSE

Thanks