SMTP relay check (test) (Postfix)

I want to add item/trigger to Zabbix to check that some server A can send mails via mail server B (SMTP relay) **without sending on real/fake email** and I haven't a problem with firewall/iptables rules and postfix rules. Mail service belongs to me and I use Postfix / CentOS 7. How can I do it? I know that I can send real mail via mailx or netcat, the command will be something like this: echo "Test message" | mailx -v -r "from@some.com" -s "Some subject" -S smtp="mail-server-b.com:25" to@some.org and parse result but I don't want to do it.