Emails can not be sent or received by the mail server due to ports limitations

Symptoms

Emails can not be sent or received by the mail server in ispmanager 6.

Causes

Limitations on the mail server ports.

For the mail server to work correctly, ports 110, 143, 993, 995, 587, 465 and 25 must be available on the server.

Use the telnet and nmap utilities to check if the ports are available for connection.

Telnet check

Using the telnet utility, it is possible to check port availability from a remote machine by executing the command:

telnet <ip address> <port>

For example, to check the availability of inbound connections (port 110) on the mail server at 172.31.97.58, run the following command:

telnet 172.31.97.58 110

If the port connection to the server is available, the command will output the following:

Trying 172.31.97.58...
Connected to 172.31.97.58.
Escape character is '^]'.
+OK Dovecot (Ubuntu) ready

If the port connection is unavailable, the command will return an error: «Unable to connect to remote host: Connection refused». 

To check the availability of outbound connections, run a command on your server. For example:

telnet smtp.gmail.com 25

If the connection is successful, the command output will be the following:

Trying 142.251.1.109...
Connected to smtp.gmail.com.
Escape character is '^]'.
220 smtp.gmail.com ESMTP 2adb3069b0e04-52fd5c2931asm880471e87.240 - gsmtp

If the port connection is unavailable, the command will return an error:  «Network is unreachable».

Nmap check

Using the nmap utility, it is possible to check port availability from a remote machine by running the command:

nmap <ip address> -p<port>

For example, to check the availability of inbound connections (port 110) on the mail server at  172.31.97.58, run the following command:

nmap 172.31.97.58 -p110

If the port connection to the server is available, the command output will show the open status (column STATE):

Starting Nmap 7.80 ( https://nmap.org ) at 2024-01-01 11:11 MSK
Nmap scan report for isptest.ru (172.31.97.58)
Host is up (0.014s latency).
PORT	STATE SERVICE
110/tcp open  pop3
Nmap done: 1 IP address (1 host up) scanned in 0.05 seconds

The filtered status indicates that the port connection is restricted from the server side. The closed status indicates that the port is not listened to by the service or port connection is restricted at the network level from the server side.

Additionally, the -Pn key, which is specified instead of a specific port, can be used to check the status of all listening ports:

$ nmap 172.31.97.58 -Pn
Starting Nmap 7.80 ( https://nmap.org ) at 2024-01-01 11:11 MSK
Nmap scan report for isptest.ru (172.31.97.58)
Host is up (0.071s latency).
Not shown: 982 closed tcp ports (conn-refused)
PORT     STATE    SERVICE
21/tcp   filtered ftp
22/tcp   filtered ssh
25/tcp   open     smtp
53/tcp   open     domain
80/tcp   open     http
110/tcp  open     pop3
111/tcp  open     rpcbind
143/tcp  open     imap
443/tcp  open     https
465/tcp  open     smtps
587/tcp  open     submission
873/tcp  open     rsync
993/tcp  open     imaps
995/tcp  open     pop3s
1100/tcp filtered mctp
1500/tcp open     vlsi-lm
3306/tcp open     mysql
5432/tcp open     postgresql