Python Issue - 'str' object has no attribute 'read_until'

Hi,

I'm trying to write a little script that will automate the process of checking to see if an SMTP server is acting as an open relay. I can't seem to get beyond the error "'str' object has no attribute 'read_until'" when I issue the tn.write_until statement. I probably have issues with the tn.write as well, but can't get passed this one. I've tried converting the string I'm sending to binary a number of different ways, but I get the same error. It's likely something right in front of me, but I can't see it, and I've dug around finding no solution on the net.

If anyone has any suggestions, I would greatly appreciate them.

Here is what I have now (though it has changed numerous times)
import telnetlib
import sys
import time


def main():
    host = ''
    port = ''
    tn = ''
    if len(sys.argv) < 2:
        prg = sys.argv[0]
        print('Usage: ' + prg + ' <filename> [port]')
        print('If port is omitted then default SMTP port 25 is assumed')
        sys.exit
    elif len(sys.argv) == 2:
        host = sys.argv[1]
        port = '25'
    elif len(sys.argv) == 3:
        host = sys.argv[1]
        port = sys.argv[2]

    try:
        tn = telnetlib.Telnet(host, port)
        time.sleep(3)
    except:
        print('Unable to connect to: ' + host + ' on port: ' + port)
        sys.exit
    tn.read_until('220')
    tn.write('helo mynet.com')
    tn.read_until('250')
    tn.write('mail from:<phantom@mynet.com>')
    tn.read_until('250')
    tn.write('rcpt to:<xxxxx@xxxxxx.com>')
    tn.read_until('250')
    tn.write('data')
    tn.read_until('354')
    tn.write('Subject: This Could be an Open Relay')
    tn.write('If this message is received, the SMTP server is an open relay')
    tn.write('.')
    tn.read_until('250')
    tn.close()

The traceback is:
Usage: ./ckOpenRelay.py <filename> [port]
If port is omitted then default SMTP port 25 is assumed
Unable to connect to:  on port:
Traceback (most recent call last):
  File "./ckOpenRelay.py", line 58, in <module>
    main()
  File "./ckOpenRelay.py", line 41, in main
    tn.read_until(b'220')
AttributeError: 'str' object has no attribute 'read_until'

Thank you,