收到的电子邮件的last_modified_time和datetime_sent有什么区别？

Question

以下代码：

#!/usr/bin/env python
from exchangelib import ServiceAccount, Configuration, Account, DELEGATE

import os
import logging
from config import cfg
logging.basicConfig()

credentials = ServiceAccount(username=cfg['imap_user'],
                             password=cfg['imap_password'])

config = Configuration(server=cfg['imap_server'], credentials=credentials)
account = Account(primary_smtp_address=cfg['smpt_address'], config=config,
                  autodiscover=False, access_type=DELEGATE)
unread = account.inbox.filter(is_read=False)   # returns unread emails
for msg in unread:
    print("last_modified_time={}".format(msg.last_modified_time))
    print("datetime_sent     ={}".format(msg.datetime_sent))
    print("#" * 80)

给我输出

last_modified_time=2017-08-01 09:37:56+00:00
datetime_sent     =2017-08-01 09:37:15+00:00
################################################################################
last_modified_time=2017-07-31 14:41:55+00:00
datetime_sent     =2017-07-31 14:40:34+00:00
################################################################################

为什么datetime_sent在last_modified_time之前？是发件人电子邮件服务器的时间（因此可能是什么）？之后是否由接收服务器修改了电子邮件？如何修改收到的电子邮件？标志修改（例如设置为＆＃34;读取＆＃34;）是否也在更改last_modified_time？