post_upgrade() {
    if [ $(vercmp $2 6.4.2p1-1) -lt 0 ] ; then
        cat << EOF

The configuration syntax changed in OpenSMTPD 6.4.
Please refer to https://poolp.org/posts/2018-05-21/switching-to-opensmtpd-new-config/
as well as smtpd.conf(5) for the new syntax.

EOF
    fi
}
