[CentOS] Mail server with both postfix and sendmail installed, active MTA gets switched.

Sat Jan 5 04:29:00 UTC 2019
Lamar Owen <lowen at pari.edu>

Ok, so a bit of a long subject line there...

I'm running an email server using postfix, but a critical third-party 
package being used requires 'sendmail' the package.  No, I can't 
uninstall that package, please don't suggest that, and I don't have 
control of its requires.  So both postfix and sendmail are installed, 
and that's just the way it has to be.  Amavisd-new and clamav/clamd, 
from EPEL are also installed.

I have had updates twice now switch the active MTA to sendmail, and I 
have to manually 'systemctl disable sendmail; systemctl stop sendmail' 
and 'systemctl enable postfix;systemctl start postfix' afterwards.  I am 
trying to find out what package update caused this to switch; or which 
logfile to look at to show me the systemctl messages that show the 
enable/disable sequence and points me to the correct package to file the 
bug against.

The packages updated today when this happened were:
# cat /var/log/yum.log
Jan 04 16:42:47 Updated: clamav-filesystem-0.101.0-1.el7.noarch
Jan 04 16:42:48 Updated: clamav-lib-0.101.0-1.el7.x86_64
Jan 04 16:42:48 Updated: clamav-update-0.101.0-1.el7.x86_64
Jan 04 16:42:48 Updated: clamd-0.101.0-1.el7.x86_64
Jan 04 16:42:48 Updated: clamav-server-systemd-0.101.0-1.el7.x86_64
Jan 04 16:42:48 Updated: clamav-scanner-systemd-0.101.0-1.el7.x86_64
Jan 04 16:42:49 Updated: clamav-0.101.0-1.el7.x86_64
Jan 04 16:42:54 Updated: clamav-data-0.101.0-1.el7.noarch
Jan 04 16:42:55 Updated: python2-certbot-apache-0.29.1-1.el7.noarch
Sendmail hasn't had updated packages in a long time, since August 2017.  
None of the package scriptlets from the above packages look like they 
would affect the sendmail setup.  So, I'm looking for suggestions on 
where to start looking for the data to ferret this out; no clues are in 
/var/log/messages or /var/log/yum.log.