[CentOS] yum update (first in a long time) - EXIM environment variable warning

Me again,

Another problem I've found since doing my yum update is that EXIM is now 
showing the following error, on startup, and repeatedly in main.log

[root at ollie2 ~]# exim -bt   localpart at example.com
2016-05-06 09:27:04 WARNING: purging the environment.
 Suggested action: use keep_environment and add_environment.

...............

Googling says that to fix this all I need to do is add

keep_environment

to exim.conf.  What I'm struggling to find out is why it's suddenly needed and 
what the effects are