smtplib requires that messages being sent be in ascii, and throws an exception otherwise.  
Python 2.6 doesn't require this.  Here's the diff where it was introduced:
Lib/smtplib.py?rev=59102&r1=58495&r2=59102">http://svn.python.org/view/python/branches/py3k/Lib/smtplib.py?rev=59102&r1=58495&r2=59102

Is there a good reason for this?  I use python for a webstore, and send out emails for folks 
with multibyte names (for instance, if a name has an umlaut).

Here's a code snippit + exception:

Python 3.0rc3 (r30rc3:67312, Nov 22 2008, 18:45:57) 
[GCC 3.4.6 [FreeBSD] 20060305] on freebsd6
Type "help", "copyright", "credits" or "license" for more information.
>>> import smtplib
>>> server = smtplib.SMTP("localhost")
>>> server.sendmail("gus@flyingmeat.com", "gus@flyingmeat.com", "Ümlaut")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/mu.org/home/gus/unix/python3/lib/python3.0/smtplib.py", line 713, in sendmail
    (code,resp) = self.data(msg)
  File "/home/mu.org/home/gus/unix/python3/lib/python3.0/smtplib.py", line 481, in data
    self.send(q)
  File "/home/mu.org/home/gus/unix/python3/lib/python3.0/smtplib.py", line 305, in send
    s = s.encode("ascii")
UnicodeEncodeError: 'ascii' codec can't encode character '\xdc' in position 0: ordinal not in 
range(128)

Is there a workaround or a new way of using it?  I couldn't seem to find it.

Thanks!

> Is there a good reason for this?

Most definitely. In Python 2.x, the string literal denotes
a byte string, whereas in 3.x, it is a character string.
It's not possible to send a character string directly over
the network; try encoding it.

It might be considered a bug that sendmail accepts a string
at all as long as the string only consists of ASCII characters;
it should reject such strings as well.

Encoding the message first doesn't work either:

>>> import smtplib
>>> server = smtplib.SMTP("localhost")
>>> server.sendmail("gus@flyingmeat.com", "gus@flyingmeat.com", "Ümlaut".encode("UTF-8"))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/mu.org/home/gus/unix/python3/lib/python3.0/smtplib.py", line 713, in sendmail
    (code,resp) = self.data(msg)
  File "/home/mu.org/home/gus/unix/python3/lib/python3.0/smtplib.py", line 477, in data
    q = quotedata(msg)
  File "/home/mu.org/home/gus/unix/python3/lib/python3.0/smtplib.py", line 157, in quotedata
    re.sub(r'(?:\r\n|\n|\r(?!\n))', CRLF, data))
  File "/home/mu.org/home/gus/unix/python3/lib/python3.0/re.py", line 165, in sub
    return _compile(pattern, 0).sub(repl, string, count)
TypeError: can't use a string pattern on a bytes-like object

Should a check be done in data() first, before it tries to try string operations on it?

I see. It seems Python 3.0 just won't support that usage, then.

You still should be able to use MIME to send non-ASCII characters.

For completeness, if anyone runs across this in the future, the following seems to work for sending utf-8 mail 
in python 3:

import smtplib
import email.mime.text

msg = email.mime.text.MIMEText("Ümlaut", _charset="UTF-8")

smtp = smtplib.SMTP('localhost')
smtp.sendmail('gus@flyingmeat.com', 'gus@flyingmeat.com', "Subject: This is your mail\n" + msg.as_string())
smtp.quit()

We might want to add the workaround to docs or even to extend smtplib to
support it.