[CentOS] Setting up sendmail to mail system messages on CentOS 6 and 7

Hi all,

I've previously configured sendmail to send system messages on selected
servers running CentOS 6. Usually they just kinda' work out of the box. No
problems to speak of.

However most of my CentOS 7-servers are rather flakey when it comes to
sendmail. Most of the time they just won't send mails for whatever reason. 
Further, most of the time I can get it working, it's usually about a
firewall rule not set, the firewall not having been reloaded or some simple
such.

There is however this one server that just won't play along.
Daemons are up and restarted to pick up any changes, hostnames set etc. but
still nothing. 
I'm at wits end with this one.

Here is a typical guide I've used for CentOS  6, and that I've used for the
problem server above as well as for other CentOS 7-machines.
<https://www.jeffgeerling.com/blogs/jeff-geerling/configure-sendmail-centos>

Is setting up sendmail different on CentOS 7 in any way, as compared to
CentOS 6??
Anything else I should double-check?

-- 
BW,
	Sorin
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