端口 25 不会在 Digital Ocean 液滴上打开 [重复]

我正在尝试设置电子邮件服务器。我似乎无法为我打开 25 端口。我真的只是做了：

ufw disable

ufw reset

ufw default deny incoming

ufw default allow outgoing

sudo ufw allow 22/tcp

sudo ufw allow 25

sudo ufw enable

接着sudo ufw status verbose…

Status: active
Logging: on (low)
Default: deny (incoming), allow (outgoing), disabled (routed)
New profiles: skip

To                         Action      From
--                         ------      ----
22/tcp                     ALLOW IN    Anywhere                  
25                         ALLOW IN    Anywhere                  
22/tcp (v6)                ALLOW IN    Anywhere (v6)             
25 (v6)                    ALLOW IN    Anywhere (v6)   

然而，粗略一瞥 sudo netstat -tulpn | grep 听和..

tcp        0      0 127.0.0.53:53           0.0.0.0:*               LISTEN      512/systemd-resolve 
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      664/sshd: /usr/sbin 
tcp6       0      0 :::22                   :::*                    LISTEN      664/sshd: /usr/sbin 

和telnet [my server IP] 25：

telnet: Unable to connect to remote host: Connection refused

是什么赋予了？如何打开我的端口？

更新

根据这里用户的建议，我决定现在安装 Postfix 并再次检查端口是否正在被确认..

sudo netstat -tulpn | grep LISTEN
tcp        0      0 127.0.0.53:53           0.0.0.0:*               LISTEN      512/systemd-resolve 
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      664/sshd: /usr/sbin 
tcp        0      0 0.0.0.0:25              0.0.0.0:*               LISTEN      43246/master        
tcp6       0      0 :::22                   :::*                    LISTEN      664/sshd: /usr/sbin 
tcp6       0      0 :::25                   :::*                    LISTEN      43246/master     

和

telnet [my server IP] 25
Trying [my server IP]...
Connected to [my server IP]
Escape character is '^]'.
220 ubuntu-s-1vcpu-1gb-lon1-01 ESMTP Postfix (Ubuntu)

它似乎已连接，但这是否意味着端口已打开？