i have seen much more spam on secondary mail servers too. recently, i quizzed a guy i know at sendmail.com about it. he told me that the current thinking amongst mail professionals (caveat: his opinion) is to not use secondaries. my personal secondary mx server saw nothing but spam and tons of it. the spammers continually use new techniques responding to current conditions so ymmv. Damon Coursey wrote:
From my observations we get more spam delivered to our secondary mail servers
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