我想发送邮件到 Gmail 地址：
echo "This is the body of the email" | mail -s "This is the subject line" user@gmail.com
这就是我在日志中看到的
connect to gmail-smtp-in.l.google.com[142.250.27.27]:25: Connection refused
connect to gmail-smtp-in.l.google.com[2a00:1450:4025:401::1a]:25: Network is unreachable
connect to alt1.gmail-smtp-in.l.google.com[2a00:1450:4025:c03::1a]:25: Network is unreachable
connect to alt1.gmail-smtp-in.l.google.com[142.251.9.27]:25: Connection refused
connect to alt2.gmail-smtp-in.l.google.com[142.250.150.27]:25: Connection refused
Apr 9 20:04:02 ubuntu postfix/smtp[157056]: 228717CAA0: to=<user@gmail.com>, relay=none, delay=0.22, delays=0.09/0.09/0.04/0, dsn=4.4.1, status=deferred (connect to alt2.gmail-smtp-in.l.google.com[142.250.150.27]:25: Connection refused)
尝试使用 netcat 测试此目的地，结果是相同的：
ubuntu@ubuntu:~$ nc -zvn 142.251.9.27 25
nc: connect to 142.251.9.27 port 25 (tcp) …