I think RedHat Bugzilla Bug 1465910 might be the issue.
I've added a comment to this bugzilla.
Cheers
Bill
 
-----Original message-----
> From:Lamar Owen <[log in to unmask]>
> Sent: Thursday 5th October 2017 1:21
> To: Bill Maidment <[log in to unmask]>; [log in to unmask]
> Subject: Re: Mail server problem after upgrading to SL 7.4
> 
> On 10/03/2017 05:34 PM, Bill Maidment wrote:
> > Hi
> > All my upgrades to SL 7.4 worked OK, until the mail server restarted using sendmail instead of postfix. It seems that the upgrade switched off postfix in systemd and instead switched on sendmail.
> > A bit disconcerting.
> I had this issue as well with CentOS 7.4, so it's an upstream bug and 
> would need to be reported upstream.  It was more than just disconcerting 
> here, as sendmail didn't know how to deliver to Scalix (which we use, 
> and which requires 'sendmail' even though I have it configured with 
> postfix).  So I had some email delivery issues, and some bounces, during 
> the time sendmail was active.
> 
> I hadn't looked closely enough to track where the bug actually is; it's 
> probably an assumption in the sendmail package's %post scriptlet, but I 
> reserve the right to be wrong.
> 
> I posted a message to the CentOS list afterwards on the subject, but got 
> no replies.  I figured someone else would run up against it, and you did.
> 
> 
>