So you will get an error along the lines of
exim4 is requested to be installed
but
postfix is manually installed,
postfix Conflicts mail-transport-agent,
exim4 Provides mail-transport-agent.
Instead of
Error: Conflict: postfix:amd64 -> postfix:amd64=3.8.6-1build2 -> not exim4-daemon-heavy:amd64=4.97-4ubuntu4 -> not exim4:amd64=4.97-4ubuntu4 but exim4:amd64=4.97-4ubuntu4