Excessive spam, all tagged with "AAAAAA==" - is something not working?

I'm seeing an unusually high amount of obvious spam avoiding my filters.  They all contain the header:

X-Brightmail-Tracker: AAAAAA==

which suggests to me that something is not getting scanned at all?  Normally, it's a huge long random jumble after the :

Obviously it's connecting and all working, otherwise the header would not be there at all, but what might be going wrong here?