I'm not understanding your situation... both the MX servers you've listed can both receive mail separately?
Typically, MX records are used to "backup" mail. When the primary MX goes down, the e-mails are "queued" (rather than delivered), and they are delivered when the primary MX returns.
Regardless, I think I noticed a similar situation with my e-mail redundancy setup. I had all sorts of spam countermeasures on my main, primary MX--but most of these were not installed on my backup MXes. Smarter spam robots apparently would get rejected by the primary, but when connected to the secondary, would accept e-mail because of lack of adequate spam countermeasure. My configuration had backup MXes as "trusted" so my primary mail server would blindly receive spam from the backups.
If a domain has more than one MX in the dns it's likely the spammers will favour the lower priority ones because many systems are setup with a lower amount of spam protection on the backup mx and by sending to this, the email will be accepted then forwarded on with a higher chance of the spam getting through spam filters.
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