How to log smtp debug information to a file?

The smtplib prints directly to stderr, e.g. line 823 in smtplib.py:

print>>stderr, 'connect fail:', host

You'd have to either monkey patch sys.stderr before you import smtplib or smtplib.stderr before you run your mail code.

I might also suggest patching smtplib.stderr with a custom object that has a write method to wrap your logging code (if you are using the logging library for instance):

import logging
import smtp

class StderrLogger(object):

    def __init__(self):
        self.logger = logging.getLogger('mail')

    def write(self, message):
        self.logger.debug(message)

org_stderr = smtp.stderr
smtp.stderr = StderrLogger()

# do your smtp stuff

smtp.stderr = org_stderr

This question contains some useful examples of patching stderr with context managers.

Some time ago I forked smtplib, and added a logfile option (among other things). You can try that, if you want. It also has a new name, SMTP.py.

Hardly clean, but since it doesn't seem SMTP objects provide the ability to specify where debug output goes:

import sys
orig_std = (sys.stdout, sys.stderr)
sys.stdout = sys.stderr = open("/path/to/log", "a")
# smtplib stuff
sys.stdout, sys.stderr = orig_std