Do some spammers ignore MX ptrs?

[Michael Masse]

I'm offloading just Mailscanner services from an overloaded email store
machine to a new machine and am doing so by pointing the MX record for
the domain to the new machine.   The A ptr still points to the old
system so that none of the other services this machine provides get
interupted.     All legitimate email and most spam is going to the new
system like it's supposed to and it then gets relayed to the old store
machine, but I'm noticing quite a bit of spam is still being sent
directly to the old system.     Does some spam software ignore the MX
ptr and go to the A ptr instead, or is this more likely to be a DNS
cache issue on the sending systems that will hopefully clear itself out
over a few days?   

Mike