This relates to the unresolved bpo-9208 (Python 2).

SMTPHandler fails when receiving unicode strings.

Example (from msg109621):
import logging,logging.handlers
smtpHandler = logging.handlers.SMTPHandler(
mailhost=("smtp.free.fr",25),
fromaddr="from@free.fr", toaddrs="to@free.fr",
subject=u"error message")
LOG = logging.getLogger()
LOG.addHandler(smtpHandler)
LOG.error(u"accentu\u00E9")

… fails:
--- Logging error ---

Traceback (most recent call last):
  File "/usr/lib/python3.5/logging/handlers.py", line 985, in emit
    smtp.sendmail(self.fromaddr, self.toaddrs, msg)
  File "/usr/lib/python3.5/smtplib.py", line 846, in sendmail
    msg = _fix_eols(msg).encode('ascii')
UnicodeEncodeError: 'ascii' codec can't encode character '\xe9' in position 108: ordinal not in range(128)
Call stack:
  File "/tmp/x.py", line 8, in <module>
    LOG.error(u"accentu\u00E9")
Message: 'accentué'
Arguments: ()

As discussed in msg252928 and msg252931, EmailMessage/send_message should be used instead to resolve this issue.

Patch attached.

This mostly looks good to me, Vinay.

Simon: did you intentionally omit the date header, and if so why? (The smtp server normally adds one, but you can't really depend on that). Adding it would look like:

msg['Date'] = email.utils.localtime()

(Hmm. I wonder if send_message should add Date header if there isn't one...)

I omitted the date header w/o intent. Basically because I couldn't quickly figure out how to set it.

New changeset eb843115e052 by Vinay Sajip in branch '3.4':
Closes bpo-25411: Improved Unicode support in SMTPHandler.
https://hg.python.org/cpython/rev/eb843115e052

New changeset b99b3ddd0ac4 by Vinay Sajip in branch '3.5':
Closes bpo-25411: Merged fix from 3.4.
https://hg.python.org/cpython/rev/b99b3ddd0ac4

New changeset 522b5cdffd42 by Vinay Sajip in branch 'default':
Closes bpo-25411: Merged fix from 3.5.
https://hg.python.org/cpython/rev/522b5cdffd42