Sending mail through python using smtp behind proxy server

Question

I am trying to send email using smtp library in python. But the problem is due to a proxy server (requires authentication) being used in my college the code isn't working. I get the following error:

Traceback (most recent call last):
  File "mail.py", line 41, in <module>
    "10.jpg")
  File "mail.py", line 29, in mail
    mailServer = smtplib.SMTP("smtp.gmail.com", 587)
  File "/usr/lib/python2.7/smtplib.py", line 249, in __init__
    (code, msg) = self.connect(host, port)
  File "/usr/lib/python2.7/smtplib.py", line 309, in connect
    self.sock = self._get_socket(host, port, self.timeout)
  File "/usr/lib/python2.7/smtplib.py", line 284, in _get_socket
    return socket.create_connection((port, host), timeout)
  File "/usr/lib/python2.7/socket.py", line 571, in create_connection
    raise err
socket.error: [Errno 101] Network is unreachable

Here's the code:

#!/usr/bin/python
import smtplib
from email import Encoders
from email.MIMEMultipart import MIMEMultipart
from email.MIMEBase import MIMEBase
from email.MIMEText import MIMEText
import os

USER = "...."
PASSWORD = "...."

def mail(to, subject, text, attach):
    msg = MIMEMultipart()
    msg['From'] = USER
    msg['To'] = to
    msg['Subject'] = subject

    msg.attach(MIMEText(text))

    part = MIMEBase('application', 'octet-stream')
    part.set_payload(open(attach, 'rb').read())
    Encoders.encode_base64(part)
    part.add_header('Content-Disposition','attachment; filename="%s"' %os.path.basename(attach))
    msg.attach(part)

    mailServer = smtplib.SMTP("smtp.gmail.com", 587) 
    mailServer.ehlo() 
    mailServer.starttls() 
    mailServer.ehlo() 
    mailServer.login(USER, PASSWORD) 
    mailServer.sendmail(USER, to, msg.as_string()) 
    mailServer.close() 

mail("yrishabh24@gmail.com",
   "BB Testing",
   "This is a email sent with python",
   "10.jpg")

How can I fix this?