Thousands of ssh probes

[Ian Smith]

In freebsd-questions Digest, Vol 300, Issue 10, Message: 6
On Fri, 05 Mar 2010 16:07:29 +0000 Matthew Seaman <m.seaman at infracaninophile.co.uk> wrote:
 > On 05/03/2010 15:51:52, Randal L. Schwartz wrote:
 > > The spamtrap is a shiny object for spam, and anything that goes there gets
 > > blocked for an hour from hitting the low port.  I presented this at a
 > > conference once.
 > 
 > Having an IPv6-only high-mx seems to terminally confuse most spambots...

I understand why IPv6 would confuse them, but don't follow why higher 
numbered MXs would be more attractive to them in the first place?

Are they assuming a 'secondary' MX will be more likely to accept spam?

cheers, Ian