我正在尝试使用简单的命令从Ubuntu计算机向我的Gmail收件箱发送电子邮件
USER@MACHINE:~$ echo "This is the body of the email" | mail -s "This is the subject line" RECIPIENT
我从未收到过电子邮件，而/var/log/mail.log中的日志文件显示了一些我不知道如何解决的错误。有人可以就此提出建议吗？以下是mail.log的内容：
USER@MACHINE:~$ echo "This is the body of the email" | mail -s "This is the subject line" RECIPIENT
USER@MACHINE:~$ sudo tail /var/log/mail.log
[sudo] password for USER:
Aug 24 11:00:28 MACHINE postfix/error[4862]: C565E62BF3: to=<RECIPIENT>, relay=none, delay=949, delays=949/0.08/0/0.03, dsn=4.4.1, status=deferred (delivery temporarily suspended: connect to ALT2.ASPMX.L.GOOGLE.COM [64.233.186.27]:25: Connection refused)
Aug 24 11:06:24 MACHINE postfix/pickup[4749]: 9179D62BF5: uid=1000 from=<USER>
Aug 24 11:06:24 MACHINE postfix/cleanup[4868]: 9179D62BF5: message-id=<20170824160624.9179D62BF5@MACHINE>
Aug 24 11:06:24 MACHINE postfix/qmgr[4750]: 9179D62BF5: from=<USER@SUB.DOMAIN.NET>, size=335, nrcpt=1 (queue active)
Aug 24 11:06:25 MACHINE postfix/smtp[4870]: connect to ASPMX.L.GOOGLE.COM[2607:f8b0:4002:c06::1a]:25: Network is unreachable
Aug 24 11:06:25 MACHINE postfix/smtp[4870]: connect to ASPMX.L.GOOGLE.COM[74.125.21.26]:25: Connection refused
Aug 24 11:06:25 MACHINE postfix/smtp[4870]: connect to ALT2.ASPMX.L.GOOGLE.COM[64.233.190.27]:25: Connection refused
Aug 24 11:06:25 MACHINE postfix/smtp[4870]: connect to ALT1.ASPMX.L.GOOGLE.COM[173.194.210.27]:25: Connection refused
Aug 24 11:06:25 MACHINE postfix/smtp[4870]: connect to ALT2.ASPMX.L.GOOGLE.COM[2800:3f0:4003:c01::1a]:25: Network is unreachable
Aug 24 11:06:25 MACHINE postfix/smtp[4870]: 9179D62BF5: to=<RECIPIENT>, relay=none, delay=1.2, delays=0.11/0.05/1/0, dsn=4.4.1, status=deferred (connect to ALT2.ASPMX.L.GOOGLE.COM[2800:3f0:4003:c01::1a]:25: Network is unreachable)