Ignore the last message - this one should work.
Here's what's working for me. You have to have 2 working MX records to
do this. The idea is that it does DEFER on the lowest one and the higher
one accepts it. Gets rid of 90% of them. Probably someone else could
write something better.
defer message = DEFER - One word message - LC=$body_linecount
domains = @mx_primary
!regex = \N\x20\w\N
condition = ${if < {$body_linecount}{3}{yes}{no}}