"Out of memory as I tried to allocate 8392749 bytes" ?

I just noticed that there were over 20 of these entries in maillog over a
three hour period.  That's almost one for each incoming e-mail on this
small system.

I'm running BSDI 3.1 on a PPro 200 with 128 MB RAM and have set up
system-wide procmail spam filtering (which appears to be working).

Any ideas about what is happening?

TIA
- drh