>>> import smtplib
>>> help(smtplib.SMTP.__init__)
Help on method __init__ in module smtplib:

__init__(self, host='', port=0, local_hostname=None) unbound smtplib.SMTP method
    Initialize a new instance.
    
    If specified, `host' is the name of the remote host to which to
    connect.  If specified, `port' specifies the port to which to connect.
    By default, smtplib.SMTP_PORT is used.  An SMTPConnectError is raised
    if the specified `host' doesn't respond correctly.  If specified,
    `local_hostname` is used as the FQDN of the local host.  By default,
    the local hostname is found using socket.getfqdn().

>>> help(smtplib.SMTP.connect)
Help on method connect in module smtplib:

connect(self, host='localhost', port=0) unbound smtplib.SMTP method
    Connect to a host on a given port.
    
    If the hostname ends with a colon (`:') followed by a number, and
    there is no port specified, that suffix will be stripped off and the
    number interpreted as the port number to use.
    
    Note: This method is automatically invoked by __init__, if a host is
    specified during instantiation.

class BindableSMTP(smtplib.SMTP):
    def __init__(self, host='', port=0, local_hostname=None, *args, **kwargs):
        self.interface = kwargs.get('interface', None)
        smtplib.SMTP.__init__(self, host, port, local_hostname)

    def connect(self, host='localhost', port = 0):
        if not port and (host.find(':') == host.rfind(':')):
            i = host.rfind(':')
            if i >= 0:
                host, port = host[:i], host[i+1:]
                try: port = int(port)
                except ValueError:
                    raise socket.error, "nonnumeric port"
        if not port: port = SMTP_PORT
        if self.debuglevel > 0: print>>stderr, 'connect:', (host, port)
        msg = "getaddrinfo returns an empty list"
        self.sock = None
        for res in socket.getaddrinfo(host, port, 0, socket.SOCK_STREAM):
            af, socktype, proto, canonname, sa = res
            try:
                self.sock = socket.socket(af, socktype, proto)
                #added lines
                if self.interface:
                    self.sock.bind((self.interface, 0))
                if self.debuglevel > 0: print>>stderr, 'connect:', sa
                self.sock.connect(sa)
            except socket.error, msg:
                if self.debuglevel > 0: print>>stderr, 'connect fail:', msg
                if self.sock:
                    self.sock.close()
                self.sock = None
                continue
            break
        if not self.sock:
            raise socket.error, msg
        (code, msg) = self.getreply()
        if self.debuglevel > 0: print>>stderr, "connect:", msg
        return (code, msg)

conn = BindableSMTP('smtp.server.com',587, interface='123.123.123.123')