从 16.04 升级后，我无法在 Ubuntu 20.04 上安装 Postfix，之前使用的是 sendmail。尝试安装时出现以下输出后缀使用apt：
Job for postfix.service failed because the control process exited with error code.
See "systemctl status postfix.service" and "journalctl -xe" for details.
invoke-rc.d: initscript postfix, action "restart" failed.
● postfix.service - Postfix Mail Transport Agent
Loaded: loaded (/etc/systemd/system/postfix.service; enabled; vendor preset: enabled)
Active: failed (Result: exit-code) since Thu 2020-12-24 04:56:39 PST; 6ms ago
Process: 1034683 ExecStart=/usr/sbin/postfix start (code=exited, status=1/FAILURE)
Dec 24 04:56:38 Hoth systemd[1]: Starting Postfix Mail Transport Agent...
Dec 24 04:56:38 Hoth postfix/postfix-script[1034691]: fatal: the Postfix mail system is already running
Dec 24 04:56:39 Hoth systemd[1]: postfix.service: Control process exited, code=exited, status=1/FAILURE
Dec 24 04:56:39 Hoth systemd[1]: postfix.service: Failed with result 'exit-code'.
Dec 24 04:56:39 Hoth systemd[1]: Failed to start Postfix Mail Transport Agent.
dpkg: error processing package postfix (--configure):
installed postfix package post-installation script subprocess returned error exit status 1
关键似乎是：致命：Postfix 邮件系统已在运行
我努力了：
- 重新安装
- 清除
postfix - 禁用
sendmailsystemd 单元 - 清除
sendmail - 重新安装之前手动停止 postfix
看起来安装程序启动了 Postfix，然后尝试再次启动它，然后由于这种意外状态而阻塞。