[exim] Way less spam after change of ip-address?

Hi,

just a stupid question. My ISP reorganized their network lately, and I
had to change my static ip on my primary mx. After this, the server
receives almost no spam, and the little spam it gets scores so high on
Spamassassin, so that it is rejected.

Is this normal? Do spammers cache the mx-address? I'm afraid there is
something else wrong, but by reading the logs I don't see anything
wrong. There just is no spam anymore :P

Any thoughts?

Best regards,
Stian