Zabbix server tries to connect to ports 25, 143 and 80 from some monitored hosts

[robertitox]

Dear, I have a Zabbix 4.4 server running OK, but when I see the service log I notice that Zabbix server tries to contact TCP/25, TCP/80 and TCP/143 ports from some monitored hosts. Theses hosts have Zabbix agent installed in passive mode, and the templates they have ar: Template OS Linux by Zabbix Agent and Template VM Vmware.

From the service log I can see logs like these:

5038:20200914:152632.006 TCP expect network error: cannot connect to [[host1]:25]: [4] Interrupted system call
5052:20200914:152633.020 TCP expect network error: cannot connect to [[host2]:80]: [4] Interrupted system call
5025:20200914:152636.014 TCP expect network error: cannot connect to [[host3]:25]: [4] Interrupted system call
5044:20200914:152643.119 TCP expect network error: cannot connect to [[host4]:25]: [4] Interrupted system call
5025:20200914:152644.145 TCP expect network error: cannot connect to [[host4]:143]: [4] Interrupted system call

And using tcpdump on Zabbix, I can see packets trying to go to TCP/25, TCP/143 and TCP/80 remote hosts ports, taking into account that I'm using the Zabbix agent in each hosts.

Our corpofrate firewall permit port TCP/10050 and ICMP from Zabix to all monitored hosts.

What can be the problem?

Special thanks!!!

[Atsushi]

If you use simple check to check SMTP (port number 25), HTTP (port number 80), IMAP (port number 143), Zabbix server connects to each host with each port number. Will need to be allowed. If you want to check from Zabbix agent, please check using item of type Zabbix agent.