I created some triggers and add actions to send mail through SMTP.
Though I can send e-mail by connecting the zabbix server via SSH, the trigger actions always seem as 'failed' that means the warning mails cannot be sent.

In /var/log/zabbix/zabbix_server.log it says "no welcome message 220* from SMTP server [421 4.3.2 Service not available]" but the mails can be sent when I telnet to the mail server with the port 587.

What can be wrong with the configuration that the system cannot send mail automatically? I attached the screenshots by the way. Thank you for the suggestions.