sendmail/MX problem

[Jer]

Hi All

we seem to be having a problem we sendmail on one of our boxs (4.8R)


mail sent to that domains somtimes goes to the backup MX record
yet these systems are on the same subnet . so connectively isn't the issue.
the MX records are in the right order on all nameservers and the maillog 
files look OK yet mai is still going to the wrong place?

any sugestions on where to start debuging?

thankyou

Jer