python smtplib发送邮件遇到的认证问题

python的smtplib模块主要是用来发送邮件的，使用起来比较方便。

使用程序发送邮件只需要写以下几行代码就OK了：

#!/usr/bin/env python
import smtplib
s = smtplib.SMTP(mail server, port)
s.login(username, passwd)
s.sendmail(fromaddr, toaddrs, msg)

不过使用这种方法不一定总是可行，昨天用这种方式发送邮件的时候程序总是会抛异常：

  File "/usr/lib64/python2.7/smtplib.py", line 617, in login
    raise SMTPException("No suitable authentication method found.")
smtplib.SMTPException: No suitable authentication method found.

查看python smtplib.py代码

        if not self.has_extn("auth"):
            raise SMTPException("SMTP AUTH extension not supported by server.")

        # Authentication methods the server supports:
        authlist = self.esmtp_features["auth"].split()

        # List of authentication methods we support: from preferred to
        # less preferred methods. Except for the purpose of testing the weaker
        # ones, we prefer stronger methods like CRAM-MD5:
        preferred_auths = [AUTH_CRAM_MD5, AUTH_PLAIN, AUTH_LOGIN]

        # Determine the authentication method we‘ll use
        authmethod = None
        for method in preferred_auths:
            if method in authlist:
                authmethod = method
                break

        if authmethod == AUTH_CRAM_MD5:
            (code, resp) = self.docmd("AUTH", AUTH_CRAM_MD5)
            if code == 503:
                # 503 == ‘Error: already authenticated‘
                return (code, resp)
            (code, resp) = self.docmd(encode_cram_md5(resp, user, password))
        elif authmethod == AUTH_PLAIN:
            (code, resp) = self.docmd("AUTH",
                AUTH_PLAIN + " " + encode_plain(user, password))
        elif authmethod == AUTH_LOGIN:
            (code, resp) = self.docmd("AUTH",
                "%s %s" % (AUTH_LOGIN, encode_base64(user, eol="")))
            if code != 334:
                raise SMTPAuthenticationError(code, resp)
            (code, resp) = self.docmd(encode_base64(password, eol=""))
        elif authmethod is None:
            raise SMTPException("No suitable authentication method found.")

抛出异常的地方是上面代码中加粗的地方，主要是当前连接支持server并不支持[AUTH_CRAM_MD5, AUTH_PLAIN, AUTH_LOGIN]

中的任何一种认证方式，导致程序运行出现问题。

解决方案是：在初始化SMTP和login之间调用starttls()方法就可以了，完整的代码如下：

#!/usr/bin/env python
import smtplib
s = smtplib.SMTP(mail server, port)
s.starttls()
s.login(username, passwd)
s.sendmail(fromaddr, toaddrs, msg)