SMTP（Simple Mail Transfer Protocol）

制定：

First：RFC 788 in 1981

Last：RFC 5321 in 2008

端口：

TCP 25(SMTP), TCP 465/587(SMTP_SSL)

功能：

用户客户端：

发送消息：SMTP

接收和管理消息：POP3、IMAP

邮件服务器：

发送和接收消息：SMTP

说明：

SMTP仅定义了消息传输格式（如消息发送者参数），而非消息内容（如消息头和消息体）。

邮件发送模型

流程描述：

（1）     使用SMTP协议，通过MUA（Mail User Agent）提交邮件到MSA（Mail Submission Agent），然后MSA递送邮件到MTA（Mail Transfer Agent）[通常而言，MSA和MTA是同一邮件服务器上的两个不同例程，其也被称为SMTP服务器]。

（2）    MTA通过DNS在MX（Mail Exchanger Record）上查询接收者的邮件服务器。

（3）    MTA发送邮件到目标邮件服务器MTA，其间可能是两个MTA直接通信，或者经过多个SMTP服务器，最终到达MDA（Mail Delivery Agent）。

（4）    MDA直接递送或者通过SMTP协议，将邮件递送到本地MTA。

（5）    终端应用程序通过IMAP/POP3协议接收和管理邮件。

SMTP协议

SMTP由3部分构成：

• MAIL：建立发送者地址。

• RCPT：建立接收者地址。

• DATA：标志邮件内容的开始。邮件内容由两部分构成：邮件头和以空行分割的邮件体。DATA是一组命令，服务器需要做两次回复：第一次回复表明其准备好接收邮件，第二次回复表明其接收/拒绝整个邮件。

一个简单的SMTP发送示例（主要为了说明SMTP的3个部分），如sender给receiver1和receiver2发送邮件：

S：表示SMTP服务器             C：表示SMTP客户端

客户端C建立TCP连接后，服务端S通过greeting建立会话（附带服务器的FQDN（Fully Qualified Domain Name）），客户端C通过HELLO响应会话（附带客户端的FQDN）。

# 建立会话
S: 220 smtp.example.com ESMTP Postfix
C: HELO client.example.org
S: 250 Hello client.example.org, I am glad to meet you

# 发送消息
C: MAIL FROM:<[email protected]>
S: 250 Ok
C: RCPT TO:<[email protected]>
S: 250 Ok
C: RCPT TO:<[email protected]>
S: 250 Ok
C: DATA
S: 354 End data with <CR><LF>.<CR><LF>
C: From: "Sender Example" <[email protected]>
C: To: "Receiver1 Example" <[email protected]>
C: Cc: [email protected]
C: Date: Tue, 15 January 2008 16:02:43 -0500
C: Subject: Test message
C:
C: Hello Receiver1.
C: This is a test message with 5 header fields and 4 lines in the message body.
C: Your friend,
C: Sender
C: .
S: 250 Ok: queued as 12345
C: QUIT
S: 221 Bye
{The server closes the connection}

Python邮件模块

python中邮件发送使用smtplib模块，其一般流程为：初始化连接->发送邮件->退出。其中包含的类有SMTP，SMTP_SSL，LMTP。如上SMTP中的会话示例在python中对应的部分如下：

C: HELO client.example.org --- SMTP.helo([hostname]): 使用HELO向SMTP服务器标识自己。
S: 250 Hello client.example.org, I am glad to meet you
 --- SMTP.verify(address)：服务器用来验证地址的有效性，返回一个元组（250，相关地址）或者（>=400, 错误消息字符串）。
C: MAIL FROM:<[email protected]> --- low-level methods: mail()
C: RCPT TO:<[email protected]> --- low-level methods: rcpt()
C: DATA --- low-level methods: data()
C: QUIT --- SMTP.quit(): 终止会话，关闭连接。

Python Email Demos

  1 # -*- coding:utf-8 -*-
  2 from __future__ import print_function
  3 from __future__ import with_statement
  4
  5
  6 __author__ = ‘xxx‘
  7 __all__ = [‘BTEmail‘]
  8
  9
 10 import smtplib
 11 import sys
 12 import os
 13 import time
 14 import mimetypes
 15 from email import encoders
 16 from email.header import Header
 17 from email.utils import parseaddr, formataddr
 18 from email.mime.text import MIMEText
 19 from email.mime.multipart import MIMEMultipart
 20 from email.mime.image import MIMEImage
 21 from email.mime.audio import MIMEAudio
 22 from email.mime.base import MIMEBase
 23
 24
 25 # ----- The following information should be in the configuration file.
 26 from_addr = ‘[email protected]‘
 27 to_addrs = [‘[email protected]‘,  ‘[email protected]‘]
 28 cc_addrs = [‘[email protected]‘, ‘[email protected]‘]
 29 bcc_addrs = []
 30 username = ‘[email protected]‘
 31 password = ‘---‘
 32 # ------------------------------------------------------------------------------------
 33
 34
 35 class BTEmail(object):
 36     def __init__(self, host=‘smtp.sina.com‘, port=465, username=None, password=None):
 37         self.host = host
 38         self.port = port
 39         self.username = username
 40         self.password = password
 41
 42     @classmethod
 43     def _format_addr(cls, envolope_string):
 44         # parseaddr() only parses single address or string.
 45         (name, addr) = parseaddr(envolope_string)
 46         return formataddr(( 47             Header(name, ‘utf-8‘).encode(), 48             addr.encode(‘utf-8‘) if isinstance(addr, unicode) else addr))
 49
 50     @classmethod
 51     def construct_msg(cls, contentfile, contenttype=‘html‘, charset=‘utf-8‘, 　　　　　　　　　　　　　　　　　　attach_flag=False, attachpath=None):
 52         with open(contentfile, ‘rb‘) as fp:
 53             content = MIMEText(fp.read(), contenttype, charset)
 54
 55         if not attach_flag:
 56             msg = content
 57         else:
 58             # Construct root container.
 59             msg = MIMEMultipart()
 60             # Attach mail contents to the container.
 61             msg.attach(content)
 62
 63             # Attach attachments to the container.
 64             for filename in os.listdir(attachpath):
 65                 item = os.path.join(attachpath, filename)
 66                 if not os.path.isfile(item):
 67                     continue
 68                 #  Guess the content type based on the file‘s extension.
 69                 (ctype, encoding) = mimetypes.guess_type(item)
 70                 if ctype is None or encoding is not None:
 71                     ctype = ‘application/octet-stream‘
 72                 (maintype, subtype) = ctype.split(‘/‘, 1)
 73
 74                 # Load  and encode attachments.
 75                 """
 76                 # Use MIMEBase class construct attachments.
 77                 with open(item, ‘rb‘) as fp:
 78                     attachment = MIMEBase(_maintype=maintype, _subtype=subtype)
 79                     attachment.set_payload(fp.read())
 80                     encoders.encode_base64(attachment)
 81                 """
 82                 # Use concrete type class construct attachments.
 83                 if maintype == ‘text‘:
 84                     with open(item, ‘rb‘) as fp:
 85                         attachment = MIMEText(fp.read(), _subtype=subtype)
 86                 elif maintype == ‘image‘:
 87                     with open(item, ‘rb‘) as fp:
 88                         attachment = MIMEImage(fp.read(), _subtype=subtype)
 89                 elif maintype == ‘audio‘:
 90                     with open(item, ‘rb‘) as fp:
 91                         attachment = MIMEAudio(fp.read(), _subtype=subtype)
 92                 else:
 93                     with open(item, ‘rb‘) as fp:
 94                         attachment = MIMEBase(_maintype=maintype, _subtype=subtype)
 95                         attachment.set_payload(fp.read())
 96                         encoders.encode_base64(attachment)
 97
 98                 # Set attachment header.
 99                 attachment.add_header(‘Content-Disposition‘, ‘attachment‘, filename=filename)
100                 msg.attach(attachment)
101         return msg
102
103     @classmethod
104     def make_envolope(cls, msg, from_addr, to_addrs, subject=‘A Simple SMTP Demo‘):
105         if msg:
106             msg[‘From‘] = cls._format_addr(u‘BT<%s>‘ % from_addr)
107             # msg[‘To‘] should be a string, not a list.
108             msg[‘To‘] = ‘,‘.join(to_addrs)
109             msg[‘Cc‘] = ‘,‘.join(cc_addrs)
110             msg[‘Bcc‘] = ‘,‘.join(bcc_addrs)
111             msg[‘Subject‘] = Header(u‘主题<%s>‘ % subject, ‘utf-8‘).encode()
112
113     def send_mail(self, from_addr, to_addrs, msg):
114         smtp_server = smtplib.SMTP_SSL()
115         try:
116             #smtp_server.set_debuglevel(True)
117
118             RECONNECT = 10      # Reconnect times.
119             for i in xrange(RECONNECT):
120                 try:
121                     smtp_server.connect(self.host, self.port)
122                 except smtplib.SMTPConnectError:
123                     if i < RECONNECT:
124                         print(‘Connecting to smtp server...‘)
125                         time.sleep(6)
126                         continue
127                     else:
128                         sys.exit(‘Can not connect to smtp server.‘)
129                 except smtplib.SMTPServerDisconnected:
130                     sys.exit(‘SMTP server is lost.‘)
131                 else:
132                     break
133
134             try:
135                 smtp_server.starttls()
136             except smtplib.SMTPException:
137                 print(‘SMTP server does not support the STARTTLS extension.‘)
138
139             try:
140                 smtp_server.login(username, password)
141             except smtplib.SMTPAuthenticationError:
142                 print(‘Username or Password is incorrect.‘)
143
144             try:
145                 refusal_dict = smtp_server.sendmail(from_addr,  to_addrs, msg.as_string())
146             except smtplib.SMTPSenderRefused:
147                 sys.exit(‘SMTP server does not accept the %s.‘ % from_addr)
148             except smtplib.SMTPDataError:
149                 sys.exit(‘SMTP server replied with an unexpected error code .‘)
150             else:
151                 if refusal_dict:
152                     print(‘*****SMTP server rejects the following recipients:*****‘)
153                     print(list(refusal_dict.items()))
154                     print(‘*******************************************************‘)
155         finally:
156             smtp_server.quit()
157
158
159 def main():
160     # Send text/plain email.
161     """
162     email_obj = BTEmail(host=‘smtp.sina.com‘, port=465, username=username, password=password)
163     msg = email_obj.construct_msg(r‘content\mail.txt‘, ‘plain‘, ‘utf-8‘)
164     email_obj.make_envolope(msg, from_addr, to_addrs, subject=‘This is a plain text mail.‘)
165     email_obj.send_mail(from_addr, to_addrs + cc_addrs, msg)
166     """
167
168     # Send text/html email.
169     """
170     email_obj = BTEmail(host=‘smtp.sina.com‘, port=465, username=username, password=password)
171     msg = email_obj.construct_msg(r‘content\mail.html‘, ‘html‘, ‘utf-8‘)
172     email_obj.make_envolope(msg, from_addr, to_addrs, subject=‘This is a html mail.‘)
173     email_obj.send_mail(from_addr, to_addrs + cc_addrs, msg)
174     """
175
176     # Send email with attachments.
177     email_obj = BTEmail(host=‘smtp.sina.com‘, port=465, username=username, password=password)
178     msg = email_obj.construct_msg(r‘content\mail.html‘, ‘html‘, ‘utf-8‘, attach_flag=True, attachpath=r‘attachment‘)
179     email_obj.make_envolope(msg, from_addr, to_addrs, subject=‘A html mail with attachments.‘)
180     email_obj.send_mail(from_addr, to_addrs + cc_addrs, msg)
181
182
183 if __name__ == ‘__main__‘:
184     main()