This mail never gets delivered. Any ideas why?

[Νίκος Γκρ33κ]

I'am receiving this now after some tries:

A problem occurred in a Python script. Here is the sequence of function calls leading up to the error, in the order they occurred.

 /home/nikos/public_html/cgi-bin/metrites.py in ()
    139         else:
    140                 sp = subprocess.Popen(['mail', '-f', FROM, '-s', 'Mail from Guest', 'support at superhost.gr'], stdin=subprocess.PIPE)
=>  141                 sp.communicate( MESSAGE )
    142                 status = sp.wait()
    143                 if status:
sp = <subprocess.Popen object>, sp.communicate = <bound method Popen.communicate of <subprocess.Popen object>>, MESSAGE = 'kdsjfksdjkfjksdjfs\r\n\t'
 /opt/python3/lib/python3.3/subprocess.py in communicate(self=<subprocess.Popen object>, input='kdsjfksdjkfjksdjfs\r\n\t', timeout=None)
    901                 if input:
    902                     try:
=>  903                         self.stdin.write(input)
    904                     except IOError as e:
    905                         if e.errno != errno.EPIPE and e.errno != errno.EINVAL:
self = <subprocess.Popen object>, self.stdin = <_io.BufferedWriter name=5>, self.stdin.write = <built-in method write of _io.BufferedWriter object>, input = 'kdsjfksdjkfjksdjfs\r\n\t'
TypeError: 'str' does not support the buffer interface 
      args = ("'str' does not support the buffer interface",) 
      with_traceback = <built-in method with_traceback of TypeError object>