They are two different versions. Slack includes sendmail by default.
The problem is both DNS and your config files.
DNS is wrong since it can not find the host opermail.
You could try using the ip vs the domain name as a test.
You do not need an ISP to send and receive mail if port 25
Have you compared /etc/hosts file?
If you want to relay through you ISP you can set sendmail to
relay to that server.