注意：本文仅供实验学习，请勿故意发送垃圾骚扰邮件。
以QQ邮箱为例

1. 查询邮件服务器地址

> nslookup -qt=MX qq.com

qq.com  MX preference = 10, mail exchanger = mx3.qq.com
qq.com  MX preference = 20, mail exchanger = mx2.qq.com
qq.com  MX preference = 30, mail exchanger = mx1.qq.com

qq.com  nameserver = ns3.qq.com
qq.com  nameserver = ns4.qq.com
qq.com  nameserver = ns2.qq.com
qq.com  nameserver = ns1.qq.com
ns1.qq.com      internet address = 101.89.19.165
ns1.qq.com      internet address = 157.255.246.101
ns1.qq.com      internet address = 183.36.112.46
ns1.qq.com      internet address = 203.205.220.251
ns1.qq.com      AAAA IPv6 address = 2402:4e00:8030::115
ns2.qq.com      internet address = 123.151.66.78
ns2.qq.com      internet address = 203.205.249.143
ns2.qq.com      internet address = 121.51.160.100
ns2.qq.com      AAAA IPv6 address = 2402:4e00:8010:1::11c
ns3.qq.com      internet address = 203.205.195.94
ns3.qq.com      internet address = 112.60.1.69
ns3.qq.com      internet address = 183.192.164.81
ns4.qq.com      internet address = 125.39.46.125
ns4.qq.com      internet address = 203.205.195.104
ns4.qq.com      internet address = 203.205.221.79
ns4.qq.com      internet address = 58.144.154.100
ns4.qq.com      internet address = 59.36.132.142

2. telnet连接邮件服务器发送邮件

telnet mx1.qq.com 25

连接成功后会收到qq邮箱的欢迎信息

220 newxmmxszb63.qq.com MX QQ Mail Server.
EHLO test.tset #随便写个，你的欢迎信息
250-newxmmxszb63.qq.com
250-STARTTLS
250-SIZE 73400320
250 OK
MAIL FROM:<admin@test.test> # 发件人
250 OK.
RCPT TO:<yourqq@qq.com> # 收件人
250 OK 1
DATA # 接下来是数据
354 End data with <CR><LF>.<CR><LF>.
from: admin@test.test # 发件人信息（不写容易被判定为垃圾邮件）
to: yourqq@qq.com # 收件人
subject: Test # 主题

Hello, I am a test email. # 正文 用一个换行+一个点+一个换行来结束
.
250 OK: queued as. # 已经发送

发送后可能会进入垃圾箱。
如果对方有SPF机制，那么就无法伪装发送。